I was daydreaming about building an e-bike. I see there are some off-the-shelf frames available, but there are some modifications I'd like to make. Has anyone ever requested a change in geometry? Or even more radical, submitted a custom design to be fabricated as a one-off?
Within this forum a few of us who own the 099 frameset requested and contributed for a zero setback proprietary 099 seatpost and velobuild opened a new mold for that. We paid for the new mold to be made and also paid some more for the actual manufacturing of the seat post (was a fair deal in my opinion. Many thanks to @RDY for organizing).
I think it is possible to make modifications to existing frames etc. They'll need to make a new mold for your modifications. As a one-off it is going to be pretty expensive though.
For example the 099 seatpost mold alone cost $500 to make. Chris said we could contribute $350 from a few of us in the forum. This was besides the actual cost of manufacturing the seatpost.
TanTanCycling or yishunbike may offer the kind of service you are looking for. Most of these companies will require a certain minimum order quantity before they'll consider the project.