I have listened to the answer in the Video by Sri Mylavarapu Srinivasa Rao, a renowned pravachan karta, in Telugu speaking areas.
Yes, he did explain about mAnasika sandhyAvandanam.
He explained the following issues:
In the olden days, people used to do sandhyAvandanam, without the paraphernalia, viz., photo of Gayatri mata, flowers, etc, while they are ...
So I found this video in which a Telugu pravachan karta is answering a question and telling viewers HOW to do mAnasika sandhyAvandan:
I obtained a translation of the gist of what he is saying in this video - that yes, you can do mAnasika sandhyAvandan (he doesn't mention doing it lying down in sleep obviously). But ...