Hi, Neil. It isn't ideal, but you can do this:
PS C:\Users\cknight> Invoke-NaSsh aggr show_space aggr0
Aggregate 'aggr0'
Total space WAFL reserve Snap reserve Usable space BSR NVLOG A-SIS
139243008KB 13924300KB 1253184KB 124065524KB 0KB 146888KB
Space allocated to volumes in the aggregate
Volume Allocated Used Guarantee
vol0 112567132KB 22106972KB volume
vften 24264KB 4876KB none
Aggregate Allocated Used Avail
Total space 112591396KB 22111848KB 11318404KB
Snap reserve 1253184KB 685276KB 567908KB
WAFL reserve 13924300KB 1424664KB 12499636KB