Simple answer: no, you cannot break inside units, only at the join between the value and unit (option
allow-number-unit-breaks). Thus to allow a break within the unit here you would need to write it (or at least part of it) out by hand
Longer answer: When I wrote
siunitx version two I looked very hard at this. The 'rules' say that breaking units is bad, but sometimes it is necessary. However, it was not possible to come up with a way of allowing breaks here while maintaining the other things that
siunitx does, and in particular not breaking stuff that already worked in version one. Internally, the entire unit is inside a (non-breakable)
\hbox to let me deal with bold math, and the spacing problem cannot be solved while doing this.