Personally, I would not try it because our annual premium without the subsidy is over $20k. To pay the monthly credit card bills, I'd have to cash out investments, which would generate capital gains, which would increase my MAGI, and possibly incur taxes above the 0% rate. I haven't run the numbers to be sure, but I suspect the loss of subsidies due to the increase in MAGI would more than negate the credit card rewards.
However, I probably need to think a little harder about using seasoned Roth contributions to pay the bills, which would not count toward MAGI. That would use up tax-free money that I'm conserving for other real expenditures, but I'd get the cash back at tax refund time. That might actually work, if I could come up with enough seasoned Roth money every year.
YMMV.