Is there a value that can be assigned to my time, i.e. an hourly rate, so that I can be given a receipt to remit to CRA come Income Tax Filing time? I live in Canada.
No. The volunteering part of that question is really the answer, you are looking for nothing in return; other than a feeling of having "given back".
This would also somewhat blur the line between the employees of the charity, and their volunteers in a way that I suspect could cause some friction, for example: if you were receipted for more than they made.
The point of getting a tax deduction for charitable donations is so that the donation is effectively made out of gross salary, rather than net salary - i.e. you don't pay any tax on the donation. But when you donate your time, that's already untaxed, so it doesn't make sense for you to get a deduction.
Looking at it another way, the charity could pay you for your time, and then you could gift the gross pay back to the charity leaving the charity no worse off. The charity would have to withhold tax, so you'd receive (gross pay - taxes), you'd give the charity (gross pay), and the tax authorities would return (taxes) to you because of the deduction you could then take. Works out the same, other than the extra administrative hassle.