Not aware that a 747 ever made it safely into Stornoway - would likely be unable to turn around if it did- but they have had an A310 and (I believe) a B767 there.
MoD improvements in the 1980s were supposedly to accommodate Tristar tankers but they never appeared
AFAIK.
The limited opening hours at Stornoway might also be a problem when deciding upon diversion airfields - Inverness and Aberdeen score higher on that basis.