bfm, Do you want to have a WeaponLight for this?
Perhaps a SureFire 647 WeaponLight will fit the bill? It mounts on the front sight and is offset to the right of the barrel.
I don't quite understand your post, but if you want the above SureFire with a TailCap pressure switch, my understanding is that the Z41 LockOut TailCap is straight swap for the Tape Pressure Switch it comes with. If you want a "Click" switch, then an SW02 I believe is the option you'd need.
I strongly urge you to call SureFire Customer Service to discuss your specific weapon and your options of WeaponLights because there are many. Toll-free: 800-828-8809
Other Dealers to contact are SableCo.net or MSTN.biz
For WeaponLights, it's best to call because not only can you talk through what you want, you can also find out whether they have what you need in stock, and if not, how long it'll take to get you it.
Due to the M203, you may need a longer cable if that's the option you go for. This is possible as cable length come in inchs with 4", 6", 8" & 10" being quite common I think.
Respectfully,
Al