Instead of the custard, why not just make a key lime pie filling? I'd omit the coffee and chocolate, personally. And you might layer it with a lemon cake.
I have a friend who taught me a berry tiramisu with layered ladyfingers and berries (rasp or straw, blue not being very popular here, but, heck, why not?) with a mixture of about 125 grams of mascarpone, a tiny amount of sugar, and three separated eggs - whites beaten stiff and yolks folded into the mascarpone/sugar mixture. I imagine this might work with limes, too. Yes, leave out the coffee and chocolate - too confusing.
I am not a baker..hate to measure, but am a cooker. This is what I did.
1. Make lime curd a la lemon curd recipe (use your favourite); I was desperate, so I bought a really good quality one in a local gourmet store.
2. I made a custard mixture of eggs, yolks and whipped them while over a bain marie - be careful not to scramble. When cooled, I whipped into mascarpone and folded in a very generous amount of lime zest. I let that cool. If you want to lighten the mixture, whip in some fresh whipped cream. Refrigerate.
3. Made simple syrup. When warm still, not hot, dipped lady fingers in that. After each layer of lady fingers, I spread out some lime curd over top, then the mascarpone mixture.
4. Repeat. Top with candied limes and lime zest. Refrigerate for a couple of hours at least.
Was OK, the simple syrup needs a kick for me; lime liqueur.
Better: Orange Creamsicle Tiramisu.
Use orange liqueur mixed with simple syrup for dipping ladyfingers. Add fresh OJ or thawed frozen OJ concentrate to mascarpone mixture.
Make sure to add vanilla pods to simple syrup.
That was delicious.