Indeed. So long as there are no additional empties penned in then it is essential that passenger services accomodate these rotations. The pre-covid planned timetable enhancements are impossible to maintain without them.
Strictly speaking, you don't need any additonal empties to rotate the DMU fleet and still maintain a full HST frequency on all end-to-end services between Aberdeen/Inverness and Glasgow/Edinburgh.
The key lies in 'end-to-end', and the following services have already been acting as de-facto (whilst not exclusively) rotationary movements for years, without disrupting the usual intercity formations:
1H01 Perth to Inverurie
1A43 Perth to Inverness
1L32 Aberdeen to Perth (terminates at Dundee on Saturdays)
1H25 Dundee to Inverness (starts at Montrose on Saturdays)
Allows up to 12 individual 158s, and 8 individual 170s to rotate daily Monday-Saturday. In my experience these almost always turn up as double 158s or 170+158 combos.
It's also worth noting 1A46 - Inverness to Stonehaven (weekdays only) - it's an ex-Elgin so will certainly serve the same purpose post full HST pre-covid timetable.
More than enough to sustain the rotations required without the logistical hassle of penning in more empties.
The size of Perth and Dundee's accomodation for stabling has always proved to be a blessing for planners, especially now with the HSTs and 385s thrown into the mix.
Edit:
47827 beat me to it!