I've taken wainscoting off of my bathroom wall that seems to have been around since the house was built (1900). Under one of the walls is an exposed horsehair plaster that was done right up against the brick/studs. The plaster is very uneven as it looks like the wainscoting was used to cover it up directly and there's no finish/sealer on it. It seems to have taken some moisture damage as well as it's powdery and crumbles to the touch.
I'm looking to basically fill in what the wainscoting was covering up so I'll need to even it up with the board above it. It's not a very thick layer but because it's uneven, some parts will be thicker. I can't put greenboard drywall in because there's a gas line going up through the plaster (see red rectangle image below) and the drywall will stick too far out to line up with the window trim/sill.
Will I be able apply plaster directly or will this need to all be ripped out? Can I get away with putting up a mesh? What kind of plaster considerations should I take into account considering it's a bathroom?