It's actually much easier than you think. Don't think of it as adding a straight line to 1 cavity - rather it's done as 1 continuous flow. Use a pastry bag or squeeze bottle with a very small tip. Hold it above the mold and squeeze thinking of it more like laying down a thread instead of painting in a mold. Start outside the mold so you don't get a drop in the first cavity. Your movement shouldn't hesitate and should be fairly swift. If you've ever painted walls - think of cutting that edge that meets the ceiling or baseboard. If you're too slow the line will look messy. With a confident hand and steady hand, you get great results. What you want to keep in mind is starting outside the mold, raising up at least a few inches, and 'laying down' the line rather than being right on top of it. Practice on parchment until you feel comfortable. The uneven mold makes no difference. Good luck!
Thank you for your feedback. It's great to hear that it might not be all that hard after all. I'll start practicing the technique you suggested.