I'm trying to use a seat-post mounted rack (a Topeak MTX BeamRack) on a Specialized Tricross Comp frame. The problem is that I'm also using a Cane Creek Thudbuster ST suspension seatpost and so there is not much spare seatpost showing; I would have to move the seatpost up by 5mm to make enough space to clamp the rack to, as I've done for the picture below, but the seat is then too high for me. So I was considering trimming off the top 5mm of the frame's seat tube. The seat tube is aluminum (as is the seat post). Is this a completely stupid idea?
It would seem that the only effect trimming the seat tube would have would be to shorten the slot in the top of the seat tube that allows the seat-post clamp to work (sorry, I don't know the proper name for this, see pic below). This slot is currently 41mm long; on other aluminum frames that I've measured this can be 30-35mm long, so it seems that cutting off 5mm from this would still keep it at a decent length (approx. 36mm). The seat post has loads of extra length extending into the seat tube (I'm about 10cm above the min. insert mark), so this is not a concern. I understand that I'll invalidate the frame's warranty by making this modification. I have a cutting guide tool and I've cut many fork steerers, seat-posts, and handlebars before without any problems.
Before doing anything stupid, I wanted to ask some more knowledgeable people - so please let me know what you think. Thanks.
(BTW, I know this frame has mounts for a proper rack, which I often use, but I'd like to have the simplicity of the seatpost mounted rack for small loads and for the ease of fitting and removing it).