Most any HDTV will accept a 240p signal (Although many will hardly do a decent job of displaying it). Plasmas aren't unique in that regard and still have a fixed resolution just like a LCD has with all content not matching that having to be scaled by a scaling chip (Which doesn't care in the slightest what the screen technology is). So there's absolutely no advantage over LCD technology there.
They're generally a bit better with colors (Particularly deep blacks) and motion blur but LCD technology has also advanced greatly so those advantages aren't what they once were. And if you want to play classic games on a plasma while pillarboxed for any length of time, you will suffer from temporary image retention that sometimes can require several hours of widescreen programming to erase the ghosts. There's no way around it unless you just play for a few minutes at a time and then move on to something that's 16:9.
And despite great strides, permanent screen burn-in can still happen on a plasma. Particularly during the first 1,000 hours or so of use and if you're not following the instructions on breaking in your set.
So if you want to play classic games at their OAR on one, I'd give it careful consideration. I watch many 4:3 television shows and Academy ratio movies (The standard before widescreen processes took over in the early 1950's so I've stayed away from plasmas. Not only would I regularly be suffering from IR, I imagine that pillarboxing would've became permanent fixtures by now since I watch relatively little full screen 16:9 programming (Particularly during the winter months when motorsports takes a hiatus around the world).