I'm not the m-tx maintainer. I've just noticed that the m-tx source code is Pascal and the Pascal compiler is only avalable on a limited set of arches. This is probably the reason.
Indeed you are right, I misread the build-log and didn't notice this.It seems likely that the maintainer looked at the list of architectures freepascal was available on and used that as their architecture list, then failed to update the list as freepascal added support for new architectures (armel and armhf).
IMO that is the most reasonable thing to do, that way when new architectures are added to freepascal (hopefully we will have mips support before jessie release) m-tx will be built without any further intervention.Yes, one could set the arch to any and then wait, for which arches the built packages appear. ;-)
I've just uploaded to raspbian with the debdiff at http://debdiffs.raspbian.org/main/m/m-tx/m-tx_0.60d-5%2brpi1.debdiff . I've tested that the package builds successfully with this change on debian armel, debian armhf and raspbian armhf.
If noone objects I intend to NMU this change to sid after the wheezy release (the target distribution, version number and email address in the changelog entry will be changed for the debian upload).