13mm of clearance on an F3 and F4 BLT using a 700c x 25mm tyre, so it's doable.
AMP appear to have used an epoxy resin or Loctite Retainer backed up with a grub screw to hold the bosses on.
I've not tried this with mine, but my thinking is that if you unscrew the grub screw (2.5mm Allen key) and heat the boss above around 80º C (Hot air gun), the bonding will go powdery and fail. You could then move the boss up (Providing you've still got the 11mm of clearance to the crown) and align, grub screw and re-bond.
Otherwise, clamp-on bosses are certainly available for the F4 (which have fatter 28.6mm legs), but they might be available for F3 (25.4mm) legs too, from Brion at AMP (email@example.com
There's an ongoing AMP parts order thread here: http://www.retrobike.co.uk/forum/viewtopic.php?f=3&t=247154
Otherwise, you could always forget moving/removing the canti bosses completely and run a disc or roller brake on the front...you'd need an AMP-ISO adapter
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse