Unfortunately, the reason I did the 12 hour cooking spree is because I've only seen prepackaged white bean paste for sale (and not already baked into pastry) once, and I was in Osaka at the time.
Red bean paste gets 99.9% of the action in the States, partially because the pan-Asian groceries carry it both for Japanese desserts and for bubble tea houses; Korean patbingsoo uses red bean paste as a component too (shaved ice, red bean paste, little mochi cubes, ice cream, drizzle of green tea syrup, yum).
If you like the red variety, though, I'd imagine you could use the same general theory for a cake! I'd use white cake or pound cake instead of the spice cake, though, because red's got a much more 'hey I'm here lookin' atcha' flavor than white does. Red bean paste and strawberries are pretty traditional, though, if you gut your average daifuku mochi... ^___^