You're right about RIP 1.5 only supporting 300x200 4:3, with non-square pixels, but that can be accommodated for by scaling the screen up to 1600x1200 with a 5x6 point scaler, then downscaling (if necessary) to the appropriate vertical resolution with an interpolative algorithm or via hardware. That's what setting aspect=true and running fullscreen with DOSBox does, and if you have a 1600x1200 or 1920x1200 monitor, you can get a pixel-perfect reproduction of the original graphics.
But you probably wouldn't have to go that far: later versions of RIPTerm support SVGA modes, and I've got 2.1 using 1024x768 in DOSBox with the emulated Tseng VGA card. Since RIP is a vector format, it just renders at that resolution, and (I assume, since my 1024x768 screencaps from RIPTerm look like higher-resolution renderings of the aspect-corrected 320x200 captures of the same) adds 20% to the y-dimension when rendering to account for square pixels.
Actually, PabloDraw implements a RIP renderer -- you might be able to reimplement some of their code.