I have to agree totally with Labret
I would NOT add this feature to my sites, as I too see a probable chargeback nightmare.
The surfer could all so easily say he cancelled within 29 minutes, and there you are - your f#$ked.
I own and run 230 paysites, and I wouldnt even consider adding a 30 minute trial, unless I was running and controlling the merchant account myself.
With my own merchant account, I think I would get surfers to go to full monthly after 30 minutes using a countdown timer on the top of my members area. This would make extra $$$ for sure, but I have 24/7 customer support, so I could handle it (maybe).
I would then also detect the amount of usage to keep things factual.
But to run through a processor, with all the control on there end, all I can say is - nahhhhhhhhhhh, not for me
I think I know a little teenie bit about billing systems too, as I own
http://www.multibill.com/, which isnt a processor but a checking system of processors (and a whole lot of other nasty stuff) . It isnt ready as a commercial product yet, a I am awaiting the trials of Visa Verfied before I launch.
Good luck Clay - a bold move for sure, and maybe one worth huge $$$.