but to my knowledge a successful mass action of homeless people, part of the lumpenproletariat

1932?

I think they rolled tanks over them. Not exactly the US government's proudest moment. The homeless can basically be murdered en masse, and no one's going to move a finger to help them. Completely on their own.

Though maybe OP has a point... maybe if the GOP organized it, maybe they'd have enough value to them that they'd prevent the worst sort of reprisals. Then again, Trump could've pardoned all of the Jan. 6th imbeciles and didn't, so if I were homeless I definitely wouldn't accept any Republican promises at face value.

They have no interest when lumpen protest,

I worked for a university a few years back. And our particular department was off-campus at the time, some donated building downtown. One day I get an email sent to the entire department, asking if anyone had seen a homeless family (with kids) living out of a car in our parking lot or nearby. The police had come by, asking our office manager, who had it forwarded to everyone. Ostensibly it was "so they could get them help".

I can't imagine living like that, don't want to. But if that life could possibly be worst, like if I were writing some mind-breaking dystopian story, I think I'd add "not only homeless, but being chased off by the police where ever they go".

So I contend that if 50,000 smelly homeless people were to march on DC, that yes, people would notice. And people would be interested (note, not the positive sort of interest... the same way you're interested when the exterminator informs you that you have the most roach-infested home he's ever seen).

What can a mass action of the lumpenprol do even if it could happen?

Why, they could get themselves murdered. By the truckload. I can't claim to have any true insight as to the false justifications given after the fact, or who might get volunteered for scapegoat duty if the public has remorse. The former is likely to be some claim that they were terrorists with a dirty bomb, but the latter... woo boy, it would've been convenient if Trump were still in office.

See, that's the thing I don't get. For people who claim to be compassionate, why are they always asking for other people to suffer? Even I don't want to see thousands of homeless people mowed down by machine gun fire, or hauled off to some desolate dungeon after being beaten half to death. Do "compassionate" people just not understand what it is they're asking for?