Had another look at this and broke out the phone to take pictures.
My box does have a bottom but there is space behind to attach the bracket discussed. [You must be registered and logged in to see this link.] [You must be registered and logged in to see this link.]
Now a 10" wheel has a diameter of just under 19" so I reckon the tyre would clear the brake energy store. I don't know about a 13" wheel. The wheel doesn't just hang the tyre is pulled firmly up to the lowest projecting surface within its radius. [You must be registered and logged in to see this link.]
Any clamped bracket across the "A" frame would have to be mounted at the top of the frame as a bracket at the same level as the bottom of the frame would interfere with the brake gubbins. This also disqualifies any bracket across the rear two bolts of the ALKO fitment.
The front two bolts of the ALKO fitment have a short bracket between them underneath the A frame. I can't see it been feasible extending this bracket to form a mounting point for the wheel as it would have to be overly long to get to a point where the wheel wasn't fouling something.
I'd forgot that where the "A" frame joins the Dandy more [ shaped bracket is bolted to the outside of the "A" frame. I'm not certain where in the timeline this started or if it's not used on the smallest Dandys. Note the picture of the underneath of Phoenix's Dandy [You must be registered and logged in to see this link.] [You must be registered and logged in to see this link.]
I'd not see a problem in drilling into the base of this or into the "A" frame at this point to attach a suitable bracket to hang the spare. If I did mount a bracket for the spare I'd be concious that it's not a stainless thread so I would grease the thread and use a nylock nut and get the threaded bar drilled so a "R" clip could be inserted to stop the nut unscrewing.
Given that the most convenient place for a jack is under the cover a lot might be said for leaving the spare there as well,providing its within a cover to stop the roof getting marked. Also the more often you come into contact with the spare the more likely it is to have air in it when needed.