Thanks for all the responses.

squirtdad - Taking the extra off the rack is worth thinking about. There is a lot of material at the top and bottom of the rack - it would be a huge amount of work and not easy to make it clean and look nice (I don't have any power tools). Plus, I'm worried that it could compromise the clamp's strength, so I'd rather not do it.

jmichael - Yes, it does have rack mounts, but read the last line of my original post.

I've thought of using cranks that are 5mm shorter, which would allow me to raise the seat by that much. However, I've decided that I don't want to change that without a good reason (and it would be the most costly option). Plus, since it is a cyclocross bike, the BB is reasonably high and so the seat is already quite far above the ground and I'd prefer to not make the reach to the ground any longer when I stop the bike.

I've been measuring a few more of these slots on other aluminum bikes, and haven't found one that is more than 35mm yet, some are as short as 28mm.

Since no-one has told me that it is a stupid idea, I think I'm going to go for it and make it as clean a job as possible.