A fair published credit in exchange for permission for use (especially when it's not commercial) is by no means uncommon, Alex. Many copright holders, I've found, are suprised and delighted that you even thought to ask.
I recently paid a couple of hundred bucks to the Wylie Agency for their OK to use some lines of Borges' poetry (they hold copyright to all his work) as section openers on a three-part novel. Even that, I felt very fair, although I might just have slid by under the fuzzy rules of 'fair usage'.
And, as open-handed as Harry, Jellby, you and many others are, I'd still feel morally obliged (if not entirely legally bound) to ask the conditions for re-use of a new edition of a PD book and to meet them before making a move. I would certainly open up the idea of a fair royalty agreement in return for the worker's new input.
I don't work in the field of re-mastering and subsequently publishing PD in any way, but -- again -- I would hope those who do would have the common courtesy to follow this code. To do otherwise -- legal or no -- is bad manners to say the least.
And if the practice of commercially exploiting new editorial, text design or technical input were to become common, it might well discourage those generous workers like you who tackle this difficult task for the sheer love of it and to no personal gain other than the satisfaction of a job well done and the genuine appreciation and thanks of reaers like me.
Best wishes. Neil