Check any cards you have that you don't use (mixing purchase and balance transfers = you're gonna have a bad time) for balance transfer offers. On established cards, not uncommon for them to let you do a direct-deposit cash advance with the same promotional offer as on balance transfers. Because the banks sort of understand that money is fungible. Limits to what you can do vary by bank.
Chase: $15K per 30 days
Citi: All the way up to credit limit (and they calculate fee for you so you cannot screw that part up).
Bank of America: same as Citi
All that being said, there's no guarantee that same offers will be available (or any offers) to you when the 12-24 months on the promo rate expires, so keep that in mind with how you decide to repay. I'm in the process of winding down doing this as just a general way to borrow money at low cost - yes it did work out fairly well financially for us and equivalent to paying 3-5% interest rate, but it is stressful as more than once I've had to open a new card to roll things forward when I had offers on existing cards one month and they just vanished the next. Obviously applying for a new card is also not guaranteed, and even if you do get it, takes longer to do a balance transfer than on an existing card typically.
So anyway, if you've got cash-flow or savings to cover where you can pay off the promos on-time if it comes to that, then I'd say try it. But it is not without its perils - if you do wind up not being able to pay the balances off on time, then your non-promo-rate goes up to like 30% with this approach vs. what is highly likely to stay a lower rate with your existing HELOC. I'd probably see if I could get another promo-rate HELOC before doing this CC balance transfer thing to the tune of $300K.
