I've posted this information to Senad and Cascade Times.
A mailing list I'm on for another show had the following information regarding Paramount. I've changed the post to reflect The Sentinel:
Paramount has a dedicated line for people who want to call and ask for dvd releases and they are keeping note of how many people call to ask for these to be released. They are happy for you to call them and request that the additional seasons be released - that's what the phone line is for.
They will ask for your name, but no other details - although you may like to tell them if you are phoning from abroad - and they are very friendly :-) Paramount have meetings where they discuss future releases and the number of requests they receive is one way they judge what the demand is. So please call!
If you would like to see the remaining seasons released on dvd:
1. Call Paramount on 323 956 5000 (if you are calling from outside
the US, use 001 323 956 5000).
2. Ask for "Home Entertainment" .
3. Ask politely for Paramount to release the remaining seasons of The Sentinel.