The shaft is very strong on the 2.2KW spindle and would easily bore without weakening too much. Obviously it will but to be honest these spindles don't produce enough torque to get anywhere near being able to snap a shaft of this diameter.

I don't think it would be too hard to design some way to draw the ER collet but it would have to be a single large collet permantly installed.
Then separate tool holders machined to actually hold the cutting tools. Similiar to the Tormach system.
Problem with this approach, with this spindle, is both the relatively small diameter of the largest ER20 collet @ 13mm and the fact they will need to be balanced to handle the high spindle speeds.
The tormach system works by using the largest R8 collet @20mm and the tool holders shank match's . . BUT . . the main difference and the area I think will be hardest to over come is the balancing. Also because of the relatively small 13mm shank the tool holders will also need to be machined from high quality steel and hardened appropriatly.

The tormach systems highest spindle speeds are I believe 10K Max. The jump to higher speeds I think is where all the hard work will be.? Getting it wrong will shake the machine and very quickly wear the spindle bearings away, not to mention the shite cutting it will do.

It's do-able but far from simple.!! . . . . If it was simple and done cheaply our Asian cousins would have surely done it by now don't you think .??

There are several ATC spindles out there but the all cost a few 1000's not 100's.!!