I rode riser bars in the A race at Britannia, had a real nice time and heard not a word from the officials or other racers about it. It may have been an advantage at times, a disadvantage at others.
If you're modding a mountain bike for cross I'd do your changes in this order:
1. Tires. Schwalbe CX pro's are the only CX tire I could find in 26". Night and day difference from wider mountain tires (even fast ones). I did Britannia on fast 2.0's with clipped knobs and found them surprisingly slidey- they tended to float and wander where the narrower CX tires would cut down and hold their line. I ordered mine from Tall Tree.
2. Fork. Lock out your suspension, or switch to a rigid one.
3. Drop bars: Only worth it if your bike will fit this way and have the parts or can get 'em super cheap, or really want a drop bar mountain bike (not a bad bike to have around here actually). Otherwise, it's too expensive and saving for a CX bike is a better way to go.
It's a fun series, and people don't seem to be taking themselves too seriously out there so I wouldn't worry too much about the UCI regs. You should just get out there and race, the mods will make you faster, but you'll still have a wicked time on whatever you're running currently!