If memory serves me, the AWE32/64 series from Creative and possibly some of the Emu10k1 series cards like the Live! and PCI512 have really good midi wavetable support in hardware. The Emu10k1 does have a secondary kernel ALSA module for midi also.
This page has good info on how to set it up:
Not sure if this is what you meant but that's all I could think of.
There's also the Yamaha YMF7xx series PCI cards like the YMF724E-V series using cards that have built-in XG wavetable support systems. Not sure how much luck you'd have finding one though.
If you could get a sound card with a Wavetable header, you could hunt down a DB50XG daughterboard to attach. One card series I knew that did have these were Aureal Vortex 2 series cards like the SQ2500, but finding on might be hit or miss. Haven't seen one in years.