A while back I threw together a little VST that just shows VUs for an input signal. I'll try and get it up here as a stopgap if you like - personally, I don't use it, 'cos most of the VSTis that I use have VUs in them already.
It should definitely be possible to implement in MPT itself, though, 'cos the VUs on the "General" tab show VSTi signals already (and, unlike the channel ones, they actually show what's happening with the waveform, not just sample volume).