If I understand you correctly, you are talking about changing the font for Rests, “on a local basis” - right?

If you look in

Document Options - Fonts

you will see that the font for Rests is a document setting.

In Other Words:

It is not possible to change the Rests font “on a local basis”.

Workaround:

Use the normal rests (in the document’s font for Rests).

Do not hide the rests.

Instead, move the rests vertically waaay down below the page edge.

In that way the rests will not print, but they will still provide spacing.

To move the rests, use the plug-in Move Rests.

Use the Expression Tool to add the mensural rests as “false rests”.

Here's an idea that might work, although there are several steps. This wouldn't playback if that's a concern.

Wherever you want to enter a mensural rest enter a note of the same value. Use special tools to hide the stem. Now switch to the special tools/note shape tool. Right click on the notehead, a menu appears, choose edit. Now change the font to the mensural font, and select the appropriate character.

If it worked, then you can copy and paste this mensural character to other measures.

Jeffrey,

Are you saying these rests would not play back or that they would play back incorrectly? It is possible to mute individual notes via the Edit Frame in Speedy Entry.

In my scenario, the mensural rests are actually notes so they would play back. You're correct that they could be muted individually if playback is a concern.

Peter,
That's pretty much what I've been doing; I was just hoping for a less fiddly way. Thanks, though.
Jeffrey,
That's an intriguing idea. Not sure it saves me time on this project, but it seems like it would be adaptable to other situations.