I found another alternative solution to avoid credit cards entirely, which is applicable to my site business model:
I tested the web miner and I am getting about 0.8 kH/s in Firefox on my recent model i7 running Linux Mint 17, which
appears to be comparable to those running a downloaded miner.
Thus I assume their web miner is employing C and/or assembly code. So if their API was improved so
their affiliate widget could be employed to impose a delay of a few seconds on all visitors, the bots would also experience a similar resource cost (for as long as these altcoins are not mineable by ASICs or assuming the bots are hijacked computers that only have CPUs also).
However, the economics are two orders-of-magnitude less profitable compared to Captcha advertising revenue. Using their calculator or
this one that appears to generate about $0.02 per hour. Whereas
solvermedia.com can generate slightly less than $0.01 per Captcha view (i.e. even if user doesn't respond to the Captcha).