Hmmm, both my Blizzards (a 1230 IV and a 1260) extend quite a bit past the end of the motherboard, but now that I think about it, I have the slightly shorter A1200 motherboards that came later on, with the mouse socket on a ribbon cable instead of the motherboard itself. If you have the longer type then the accelerator will only be an inch or two longer... That's still very wide for a case though - wider than any Hi-Fi separates case and so probably wider than any HTPC case as well.
Support can be worked out if needs be. I have a block of wood and a cable tie holding my 1260 in place in my (very large) tower, and I have a piece of plastic and a strategically placed groove holding my 1230 in place in my old CD deck case. Both aren't exactly ideal but have worked perfectly for many years.