Normally if they have availability they will be willing - most hotels seem to have an amount on check-in that they charge for concierge lounge (~$50 / £50 per night seems familiar when I've been in the UK and a limited number of US hotels). I don't think that they would have a problem in taking this amount from an awards stay, but, you shouldn't expect any nights credit (you would still get the points from the money spent). Note that you don't pay for the lounge per-say, but instead pay for the concierge floor (so you also get a better room).

I think it will also depend on which type of hotel - some won't have a lounge and others, such as resorts with lounges (limited numbers) and Renaissance hotels will charge more. Availability will also impact whether you can upgrade, and don't forget some hotels have lounges open at weekends.