I'm no expert on pepakura but the paper will take the place of the glass fibre cloth to a point. A few coats of resin should soak into the paper and stiffen things up nicely. Have to say though, I'm just thinking aloud here as I've never done it myself, but if I did, that's what I'd try!
Having said that, you can also get glass fibre tissue from Halfords which is thinner and doesn't have the obvious strands of the full cloth. It's intended to be used as a final layer to give a smoother surface (without the visible strands) prior to final finishing. I've used that before in model making (in an R/C boat) to add some strength without the weight, bulk and 'texture' of the full cloth. Maybe that's worth considering too?
