I am thinking that since I have a pile of Arduino Nanos that I got for $2, even though it is overkill,
I may use one of those to sample TXA/TXB and TXC and switch the additional filters.
My plan is to use a QRP Labs relay switched LPF kit since it is already exists, is pretty much exactly what I am looking for
and only costs $16 (minus the LP filters). The individual plug in LPF kits are $US4.95 each.
It will take 5 LP filters, we only need 4, but I already have an additional one for 160m that I will figure out how to switch in as well.
I was originally planning to do that somehow anyway.
I don't plan to disable the existing uBITX LPFs as I don't care about the additional current draw or
the fraction of a dB insertion loss. The existing filters are doing something, just not enough, so adding cascading LP filters
should cover all of the bases with room to spare, except for the SSB spurs on the high bands which will be a tougher nut
P.S. I am a software guy, not a hardware guy, so it makes sense to me to use a microprocessor to solve a hardware problem ;-)