My rationale is that I've been setting 25% transparency on multiple objects instead of setting them to a colour. Now that I've reason to layer some objects, I need to switch to using a colour at 100% opacity, instead of a dynamic opacity. In retrospect, this would have provided better results regardless, so it's not an X/Y problem.

https://gitlab.com/inkscape/inbox/-/issues/7724 appears to indicate that this isn't yet possible.

It is possible from View>Command Palette

Make your selection

go the the command palette ( just press ? )

type object-set-property and hit enter (or click the entry)

A small keyboard icon then appears on the left for you to type values in.

type fill-opacity, 0.5 and hit enter.

RokeJulianLockhart

https://gitlab.com/inkscape/inbox/-/issues/7724 appears to indicate that this isn't yet possible.

Maybe this:

fill-opacity, 0.5

@inklinea, "fill-opacity, 0.5" sets it to 50% opacity, but fill-opacity, 1.0 or 1 doesn't set it to opaque:

https://youtu.be/nWRvtt8aKa0

@Aero, incredibly, this doesn't work. It still displays 25% opacity afterward, even though I'd used Control + A:

https://youtu.be/H_soSQEt4gg

Links to images can be posted, but hotlinking is not permitted.

@TylerDurden, what's "hotlinking"? Google mostly states that it's just rendering an image. (I embedded a URI via an <img> tag.) On that note, I'm somehow failing to embed those YT videos aforecited. They render in the previewer, but not the sanitised post.

If you install Simple Inkscape Scripting by Scott Pakin https://inkscape.org/id/~pakin/%E2%98%85simple-inkscape-scripting

You can paste the following python code.

It will 'pop' ( remove ) a style property of  your choosing from selected elements.

selection_list = svg_root.selected
for selected in selection_list:
    selected.style.pop('fill-opacity', 0)

RokeJulianLockhart

@Aero, incredibly, this doesn't work. It still displays 25% opacity afterward, even though I'd used Control + A: https://youtu.be/H_soSQEt4gg

The test file in your linked issue #7724 used fill-opacity.
The linked video is showing opacity is used in your file.
 

@Aero, that works for some objects, although it doesn't appear to apply recursively, inside groups (at least, of cloned elements):

https://youtu.be/oKUf7lHiyUg

Alt key in combination with the selector tool may help when selecting objects on canvas.
 

This works for me. It deletes "fill-opacity:0.75;" from the style attribute of each selected shape and recurses into selected groups.

Open the Find/Replace dialog [shift+ctrl+f]
    [Find: fill-opacity:0.75;]
    [Search In: Properties]
    [Scope: Selection]
    [Find]
    [Replace: blank]
    [Replace All]

You can just as easily remove 25% shape opacity by searching for "opacity:0.25;" or any other property you can see in the XML editor.

Edit: I moved the semicolon in the search strings.