A few additional ideas:-
If you have a welder or can/are willing to re-assemble the brakes and take it to someone with a welder, you could weld a bolt onto the allen bolt and unscrew it.
With the bottom caliper bracket bolt removed, can you tap the bracket and get it to rotate about the top (seized) bolt, this might initially try to tighten the seized bolt but if you then grip the seized bolt with a vise-grips/stillsons wrench and then tap the bracket back down to it's normal position, it might loosen the seized bolt. In other words, remove bottom caliper bracket bolt, tap bracket upwards, pivoting around the seized bolt. lock vise grips or similar onto seized bolt, than tap bracket back into it's normal position. Might need to repeat a couple of times. I know that space is tight and possibly the caliper bracket won't rotate, but if it's possible, being able to rotate the caliper bracket might give you better access to get a vise-grips onto the seized bolt.
There are special sockets/extractors available to remove e.g wheel locking bolts. They are similar to easy-outs but the tapered flutes/teeth are inside the socket and grip the outside of the bolt. They're ideal for allen bolts if you can get a suitably sized one and have room to use it. You might try a Motor Factors to see if something suitable is available. If you want to see what these are like :-
www.tengtools.com have 3/8" Drive Stud Extractors in various sizes - e.g. Stock No. ST38310 fits a 10mm bolt head but costs £25 + vat, afaik :bang:.
www.tengtools.com/r/ie/en/Service-Tools/3/8inch-Drive-10mm-Stud-Extractor-ST38310 (note prices are in Euros not £Stg )
Draper/Sealey or Laser Tools etc. might have something less costly.
Many workshops now use 'Induction Heating' on seized fasteners, very fast, can reach into tight spaces and no risk of damaging adjacent areas as might happen if using a flame/torch. If the car was mobile, you could always ask a garage if they'd use their Induction Heater on just one bolt for you.
In the past, an Oxy-Acetylene welding torch would have been used to heat the seized bolt in order to free it. This would break free a rusted fastener, would also break the bond of any thread-locking compound thast might have been used. As already suggested, a gas blowtorch might work, see if you can get one that runs on Mapp Gas - this burns hotter, otherwise try for Propane - iirc this burns hotter than Butane.
I realise access might be difficult, but if you can cut a slot across the head of the allen bolt using a Dremel type tool/Angle Grinder (3" diameter if you can get one)/ hacksaw, you might be able to use a large straight screwdriver bit with an impact driver. In the old days there were things called 'Drag Link' sockets - these looked like extremely large screwdriver bits for extremely large screws. To cut a wide slot using a hacksaw, put 2 blades side-by-side into the frame.
I always try the simplest method first. First clean out the recess in the allen bolt head. Identify the type and size of bit needed. Are you sure it's an allen bolt, could it be Torx? or maybe even RIBE? (some Fiats use RIBE fasteners). If the correct bit starts to slip and round out the recess - STOP. Use a drift and hammer to strike the head of the allen? bolt to try to shock it loose. Someone else suggested trying a slightly larger bit or a bit of a different type, this often works. But first I'd try drilling the recess in the allen bolt head a bit deeper, then tap (ok, bash) the correct size bit deeper into the recess, causing it to create a deeper hexagonal hole, this might just get a better purchase on the seized bolt. If the recess has already been rounded-out, use a hammer and drift/punch to drive displaced metal around the recess back into the recess, then tap a suitable bit into the recess. All of this tapping/ hammering tends to loosen the seized bolt.
Another possibility is to drill the head off the allen bolt - you'll likely need to also drill some of the remaining bolt shank in order to be able to remove the caliper carrier bracket - obviously need to be careful so as not to damage the threads in the carrier.
In the past, a skilled Mechanic might simply use a cold chisel at the appropriate angle on the fastener to loosen it, but I've never done such a thing :devil: (not even on column steering locks
)I suppose the modern equivalent would be to use an Air Chisel.
I second what Pugglt Auld Jock said above re. using a Stillson wrench - a much under-appreciated tool. It's 'saved my bacon' many times when nothing else worked.
Al.