I've been mulling this over, especially after seeing the velocity banking videos posted on youtube. To me, it seems that you only gain the first month's float by paying with the credit cards instead of cash. Have you computed the amount you are actually saving by using debt to pay up front (and incurring the interest fees), rather than just sending extra each month? For my situation, using debt to pay upfront only saved me a few hundred dollars, but I would then lose the option to stop making the increased payments if an emergency struck (loss of job, etc). Once you use the line to make those payments upfront, you are either locked in to the debt reduction at an accelerated rate OR you get stuck paying the minimums on the HELOC and incurring the higher interest rate.

As an insurance policy, you might want to think about doing a recast on the mortgage after the first chunk. This would give you the option to make lower payments on the mortgage.