The only reason against using glass as a side window, as far as I can think of, is safety - If your PC gets knocked over, or something bangs into it for any reason, you'd have a heckuva mess to clear up.
Acrylic is preferable because it won't shatter so easily, and it won't discolour with age, unlike perspex and some other plastic sheeting.
Putting a sheet-mirror in the bottom of your case shouldn't have any electrical problems, the silvered layer would be underneath the glass anyway, which is an insulator
Alternatively, if you can cut small sections of mirror, you could try fixing them in 'strategic' places, to reflect the light to specific darker areas.
Edit: For absolute cheapness (and recycling), if you can get a couple of those cardboard lids that's used for some 'take-away' food in the aluminium dishes. The lids are white on one side and aluminised on t'other.
Carefully wrap them in 'Clingfilm' so as not to get any wrinkles or bubbles and use them as light-'fillers'. It's a trick I piked up from a pro-photographer, he taped various sizes around his flash-guns to direct more light to specific spots. The Clingfilm would be needed to insulate the aluminium side of your ones, being in with electrics.