1. There's an all-30g Topre somewhere out there, but it won't be easy to find
2. Cherry Reds are pretty darn light, under 40g right?
3. Some regular old rubber domes are surprisingly light, like the Tsunami City Runner (44cn according to Tim Tyler's page) and the Fujitsu FKB-8720 (not the best fujitsu, but has what I consider a very light touch)
4. Modding Cherry MY/FTCS switches seems like a fun project, but I've never tried them. Anyone wanna kick me a G81 for next to nothing?
If you're looking for something off-the-shelf, I'd go with a variable weight Topre Realforce or the MX Red 'board of your choice.