I can't see why not, but the bats are quite stiff and may not fill all the gaps around them in the same way so may not be quite as effective.

We are "re-jigging" the existing Loft conversion:
The bungalow is 1961 and has two whopping great purlins running the length of the roof space. Hence, I have an usable area of 9 x 4.2 Metres with no alterations to roofing required. At the purlins the height is 1.4 Metres and at the ridge 3.7 Metres the roof is a simple gable.
Originally 2/3 of the loft was converted - we are extending this to the whole loft. The old conversion floor joists are ok for size and new ones will be added to the remainder of the unconverted area. We will be building new internal non-load bearing walls and splitting the loft into a bedroom and en-suite. We will be having a "proper" staircase installed (presently a space-saver stair with offset treads is fitted).
The new walls will be insulated to present standards and the old room perimeter walls will be replaced too.
The ceiling will be insulated to current standards as a "cold" roof - we do not intend to re-roof. The roof currently has two Velux windows in it and we will be adding a third. The chap doing the drawings for me has advised that there should be sound proofing under the floor.
I am doing as much as possible myself to keep costs down, hence the query about the cavity Batts.