I can understand rainbow warmage not being able to double progress, but it very well should be able to count as a "divine casting class", as you can cast far more divine spells than you can cast arcane ones.... so a MT of warsnake+sublime chord should work, and then you don't lose the CLs from UM....
I like the dweomerkeeper idea though.... don't really feel that the build needs the sublime chord casting (although it is spiffy), as the CL will be REALLY low, since we only have _one_ level of a base casting class (and, by RAW, thats all sublime chord CL benefits from...), so improving our extremely versatile warmage+prestige bard+cleric spell list casting is probably more efficient. after all, a warmage will not be found lacking in spells/day...
how much benefit does the build really get out of dweomerkeeper? I am not too familiar with the PrC, but i keep hearing good things about it.
Ofc you can, Straw_man, as the guide is what inspired this thread.... Im just happy you find it worth adding
EDIT: just looked up dweomerkeeper, and don't feel it is suited for the build.... have to dip contemplative and grab an item creation feat to make it work, and we won't even get the capstone pre-epic. sublime MT is looking better...