Asking Waltly is a good idea, but I don't think they will polish your welds.
XACD probably will, and they can also make an aero seat post, but it would be very difficult to source a replacement down the line if you need it.
The rest of it, from tire clearance to UDH to integration, should be pretty easy for any of the major factories accepting a one-off custom frame order to accommodate. You might be able to buy some off-the-shelf options, but they won't be much cheaper, if at all.