My 6 week old has positional plagiocephaly (flat head spot) and we were recommended to get a mimos pillow by our doctor alongside rotating the cot, tummy time, etc. We will not be using the pillow when the baby is asleep. Are there any safety concerns using this type of pressure-reducing pillow during the rest of the day for brief periods of time when he may be unsupervised?

Red Nose has the following information
Some physiotherapists and chiropractors have recommended the use of a pillow to relieve the flattening a baby’s head, a relatively common condition, known as positional plagiocephaly. This is not suitable while baby sleeps unobserved. If baby’s medical practitioner recommends the use of a pillow during unobserved sleep, it is recommended the medical practitioner puts this advice in writing.
Download Pillow Use

When flattened spots develop, these are usually reversible and rarely need significant medical or surgical intervention. The most common treatment strategies are the same as the prevention strategies.
These include
Always sleep baby on the back, not on the tummy or side.
Alternate the head position each time the baby is put down to sleep (left and right) 
As babies become more alert and interested in the environment they like to look at certain objects before falling asleep, sleeping baby at alternate ends of the cot will encourage him or her to look in different directions. Changing the position of the cot in the room may also have the same effect
When the baby is awake, minimise the time that the baby spends lying down with pressure on the same part of the head by carrying and cuddling baby in upright positions, or use a sling.
Avoid prolonged periods in car seats, strollers, swings and bouncers as this places additional pressure on the back of the head..
From birth, give baby increasing amounts of side lying and tummy time to play on a firm surface when awake and being observed by an adult but never put baby on the side or tummy to sleep.
Alternate the holding position when feeding baby i.e. hold in left arm for one feed and the right arm for the next feed.
Devices that restrict the movement of a baby or the baby’s head are not recommended.
Do not use the side sleep position to prevent positional plagiocephaly. The side position is unstable and unsafe; the babies are at a greater risk of rolling on to the tummy.
https://rednose.org.au/article/babys-head-shape