Add terms of services check to comment form : best way to go about it with Jot ?

I have a rather unusual request from a client (he’s a lawyer, this explains that) : he’d like to have users accept terms of services before being able to post a comment. Since people are not required to register to post comments can’t use WebloginPE for that purpose.

I checked Jot doc but didn’t find any parameter to that purpose.
How would you go about it ?

I was thinking add a required field (using [+form.field.customfieldname+]) and use PHx to test agains the value of the required field, but before I toy with that I was wondering if anyone had done this and how ?