has a very good chance of winning his seat back even if all parties fielded a candidate
I think this is the crux of the issue, the other parties have resigned themselves to the fact that he will in all liklihood will win the seat. He was 8,000 votes ahead in last election with 46% of the vote and neither Torys 28% or Labour 16% were anywhere close. Even the combined vote of Tories an Labour falls short of Farage's vote.
If either of these parties thought they had even an outside chance of taking his seat they would field a candidate.
They could make his abuse of the process an election issue and campaign, ironically, for reform of the system keeping Farage's stroke politics front and center.
They haven't, because he hasn't broken the rules and the rules actually suit the other MPs too in the event that they too may at some stage rely on them to do similar.
That's just the cynic in me, if you want to call yourself a parliamentarian then you can protest the laws of the land, and their processes, through election campaigning, not boycotting.