Would anyone be interested in running low% as luigi only?

(although there is a tricky jump for 1-2 SE)

Low% (Small Luigi) has been discussed multiple times in the past. The category itself would be significantly longer than Any% due to not being able to access 1-2 Secret or 2-4 Secret to get to World 5. Additionally, not enough people have gauged any interest in trying it out at all. There are many other complications with allowing it on the boards but these are the main reasons why it hasn't been considered at all.

Isn't the 1-2 secret possible as Nabbit? Not sure if that would still be considered "low%" though.

Using Nabbit kind of defeats the purpose of Low%. Low% in Mario games is "Small Mario" and Nabbit is always big.

Does nabbit count as a powerup?

No, but he's always big. So it defeats the purpose of a Small Luigi category.