Sov Null doesn't have anything to do with LP as it has no LP faucets outside a tiny trickle from ESSs so I don't know how you think an LP tax would affect it.
Isk taxation is a thing. Jump fatigue has been a thing for a while, yet new groups still seem to be taking a bite out of Sov Null. The map hasn't seen as many diverse colours since before the rise of the CFC so... I'm not seing the argument there.
In the case of tax havens, again, people already have this option right now and have taken advantage of it for years. Since it seems not everyone wants to take this approach, I guess space communism must be alive and kicking and popular. CVA happens to finance its SRP programme almost all off of isk tax on the member corps, who raise their isk off of isk tax on line members, and not a lot of people seem totally bothered by it because everyone is getting something out of it. LP tax wouldn't affect us that much at all since not a lot of people mission since we're almost perma wardecced.
Finally, what you presented is a narrow hypothetical situation and you know it. But I'll answer it anyways. It's yes and no. Yes, you are being usefull in terms of pvp and that's great. However, no, because you're not contributing to the srp pot which could be considered bad when you're happy to claim from it. But whether corps are happy to be in that situation is entirely down to them. Each corp is free to practice how they like, whether to not accept someone like that or turn a blind eye to it.
You say bottom up will be bad for null. As a nullbear, I disagree, and it seems on the whole even Goonswarm likes living off of bottom up when they happily publish the multiple billions of isk out of taxes they get and how they spent it. On that basis, since space communism is so alive and kicking despite the same issues you outline, and you've not brought up a unique problem specific to LP taxation, I still firmly endorse this mechanic.