That is completly reasonable to me, just so I know, will we get any warning if we are about to be kicked (I ask as I would love to stay in the club but with some of the big games coming around I am unsure if I will have plenty of time to get my level up in Forza 4).
But that aside, kudos to you for keeping this club going and making the Forza experience for users such as myself more enjoyable.
Generally I don't give advance warning to potentially inactive users, because I feel it'll cause more forced than genuine activity in Forza. I envision that in such a situation, the majority of users informed would quickly jump in Forza, gain a level or two, and then return to whatever games they were playing, content that they'll stay in the club for another few weeks. I don't have any problem with users doing this right now -- after all, I can't really tell -- but with the added prodding of a warning message, I think it could get to the point where barely active members make the wait extremely long for people who want in.
I do send a message when someone has been removed from the club, but obviously that's a bit late for the circumstances we're discussing here.
Another way to keep up on your club status is through the roster file I have on Google Docs. The more red highlighting by someone's gamertag (brighter being worse), the more likely I am to look further into their Forza activity to see if they've done anything recently at all when it comes time to boot/invite.