As the money is all going to a good cause and the shirts presumably could be made to order, might I suggest you get a few printed up with just 'Highlanders Rock' or other generic tag lines like 'Scotsmen Rock' or even 'Jocks Rock'. This would make them more generic and you could throw them up for sale on ebay (or even here - I'd check with the Modorators first though to see if it's allowed). I'm sure if you price them sensibly you'd get a fair few takers.

some similar examples with potential pricing are

here,

here,

and here.