So, using Rob's math:
Cheque for "work" $1000 less $350 income tax = $650 in pocket, negative $350
Donation from own pocket of $1000 x rate of refund = $350 in pocket, negative $650.
Looks like a zero proposition to me and a whole lot of hasstle...
Who wants to claim additional income on their taxes, have to make a charitable donation, wait for the receipt, apply it to their tax return and then wait for it to all wash out with CRA?
This is assuming that the "work" does not actually cost you more in taxes as your income moves out of one bracket into another. In that instance you will end up not only volunteering your time but paying for it too.
The whole idea of volunteer work is to do work for FREE.