They can see this info in the right panel:
http://awesomescreenshot.com/0325opz80d
And I guess a buyer who is close to the expiring period will click anyway on the support forum link, instead of leaving a comment…I don’t know…
Two days ago I had a support request from a buyer, in my email was clearly stated that his support has expired, but I wasn’t able to find a nice way of telling him to renew his support package. So I helped him anyway. But I agree with you, they should see this info in the comment form.