perhaps a lemon flavoured olive on top?

You could so something sour creamed based with lemon zest and dill.
Piped on top of each mini crepe.

Or what about lemon mayonnaise?

I agree with whipping the cream and then folding in zest and tarragon. If you want to add the clam flavor try adding some gelatin to the clam juice, just enough to have a week pudding like set, then whip the cream until just at soft peaks, add the zest, tarragon, and the clam mixture. Then continue whipping. You won't need a lot of the clam mixture which is good because it will probably take a few tries to get the consistance of it right.

I would omit the cracker, which I think would mess up the texture and would be too much dough considering you will also use crepes, not to mention that crackers would get soggy if the appetizers are not eaten within fifteen minutes after they are made.

This seems to be an appetizer that will take some time to assemble, and you will probably make it before guests arrive, so it is probably wise to keep in mind that textures have to be able to hold for a while. You could put little crepes in cupcake/muffin pans, fill them and then tie them shut with a couple chive stems. Then, you just need to lift them out of there with the help of a spoon to serve.

For the lemony taste, you could make a lemony mayo and include that in the filling, but I really like the editor's idea using sour cream. It may sound crazy, but I can also imagine this would work really well with cottage cheese. I would not use lemon zest unless it is very finely grated - I have the feeling I would not like to bite into a piece of zest because this seems to be a creamy textured appetizer and tough lemon zest would kind of kill it for me, but that's just me...

Personally, I would just make the original recipe and cut each rolled up crepe into inch-wide slices and put something on top of each to dress them up, sticking a toothpick into them to make them easy to handle and eat. But I am the kind of person who cares much more about the food being good and tasty than about presentation, and you might see things differently.

A greek yogurt with lemon zest might be a little more stable than whipped cream, which unless you add gelatin to it has a tendency to re-liquify at room temp. Sour cream and lemon zest like @Atthefarmersmarket suggested would also work. Although with sour cream you could probably get away with mixing in fresh lemon juice as well for a really lemony aspect.

I also like the crepe purses idea like @Viktoria suggested. Cute, eminently portable, and no escaping bits of salmon or cream!

For Crispelle: up the flour from 1/2 cup to 3/4 - 1 cup (or until it looks like pancake batter.) Use this to create a silver dollar/blini crispelle. It will be much easier to pick up.

For Salmon: to infuse flavor and liquid, poach the salmon in clam juice (which you can omit from the cream sauce) chopped shallots, chopped tarragon, and lemon zest (and a good amount of butter) add a splash of pernod if you have it.

For Cream Sauce: Creme Fraiche (Greek yoghurt will overpower the salmon) mixed with finely minced tarragon, a pinch of lemon zest, and cracked pepper.

Assemble: place poached salmon on top of "blini" and then using a piping bag--or ziplock with a corner cut off to form a tip--pipe on a dollop of the cream sauce. Top with finely chopped chives and/or grated parmesean cheese.

addendum: add 1tsp of baking soda to the crispelle batter if you're making the blini things.

Use frozen phyllo and wrap as triangles a la spanakopita instead of the crispelles!

I love the beggar's purse and blini ideas- another idea would be to make miniature savory blintzes- salmon and (lemon) cream, etc folded inside a crepe. These would be easier to store (stackable), and can be browned to serve hot.

I agree with the creme fraiche suggestion and will also suggest using wonton wrappers if you're looking to not make each little crepe individually!

Now that I think of it, you could just as well use the blini idea and stack four of them, alternating the salmon and the sour cream / creme fraiche topping and finishing with something green (a caper?) for a cute little blini tower. Use a bag to pipe the topping, otherwise assembly could turn out to be torture.

ricotta with lemon or lime zest in it

a begger's purse is a sweet idea. in my opinion, a proper bechemel, which it is, will usually set-up by itself when cold. try cutting out a little bit of the clam juice and maybe use a little gelatin to be safe. aslo, the cracker seems intrusive, try seasoned breadcrumbs for the crunch. good luck.

Wow! Thanks so much for all the comments and ideas! I ended up using an idea from one of the chefs at the restaurant I work at, I made a mustard and lemon whipped cream cheese, made the tiny crepes, and made little rollups with the crepes wrapped around the lox with the cream cheese on top. Then I baked them to make them a little crunchy. While I totally forgot that the lox is full of oil, which leaked out a little bit, the little crepes tasted great, the flavors all went really well together, and the cream cheese was pretty idiot proof.

try turning them into raviolis instead, small and easier to eat

I would mix the cooked salmon with a little of the cream sauce and fill a puff pastry round/ fold over like an empanada and bake 375 for about 20 min. Or get frozen pastry tart shells and fill them with the salmon and cream sauce and bake. Garnish with chopped chives.