Duuuuuuuude......
That means counting exactly how many beats in each measure. I did that already to get the first three measures (18/4, 26/4 and 20/4) but is this the easiest way? Am I expecting too much?

I hate to mention it, because I think it's a TERRIBLE option:
You could set the time sig to 1/whatever (4 or 8) and put ONE NOTE in each measure, and then hide/show the bar lines.
But this is a terrible idea.

I think it is much better to do one-measure per phrase (or other logical division) in non-metred scores.

Rogue, I've been doing the counting and resetting since '07, haven't figured out any other way on Finale to do it. I'd be curious as to how Bruce Ford and others using round go about setting their parameters. I also follow Bruce's way by grouping notes if they're as such in square, rather than dotting or open hole notes.

In Lilypond, you have a couple of approaches. You can (a) use \cadenzaOn to suppress automatic measure lines; and add the \bar lines explicitly; or (b) set a huge time signature (e.g., 500/8), add the \bar lines manually; and then use the \layout tag to suppress printing the time signature.

What I do is: (a) count the beats and set a time signature for each measure; and (b) use the \layout tag to suppress printing the time signature. Then the measure lines are put in automatically, and the checks at the bar lines help me to catch transcription errors.

Incidentally, I recommend using the Frescobaldi editor/previewer for editing Lilypond files.

What I do is: (a) count the beats and set a time signature for each measure; and (b) use the \layout tag to suppress printing the time signature. Then the measure lines are put in automatically, and the checks at the bar lines help me to catch transcription errors.

aaaaaaaaaaaaaaaaaaaaaand Lilypond requires counting....

Right. Optional.
But a good idea, it sounds like.

I'm trying to set some things in unmetered notation, but Finale keeps forcing in a rest or something strange.

Go to Simple Entry Options and Speedy Options, make sure to uncheck "Fill with rests at end of measure."

I, too, count the number of notes in each phrase, then fit the whole phrase within one measure. I still have to adjust the note spacing manually for the page to look good. I have experimented with entering the notes in 4/4, then hide the time signature and barlines. The note spacing was awful.

Since Finale is able to change a note-head to look like anything in a font set, I don't know why they don't offer square notes as an option. I believe there is a Gregorian font set add-on package, but it is very expensive. I also use Finale (haven't had time to experiment with Lily Pond or Gregorio). I usually follow the same style used in most modern hymnals: quarter notes for almost everything, half notes for held notes. Tenuto marks work for epismas. Still haven't figured out how to get something that resembles a quilisma. A trill symbol would work, but it doesn't lie on the correct line or space. I've seen round note scores with quilismas (such as in Bruce Ford's American Gradual) but perhaps he is not using Finale?

After all the notes are entered, hiding stems and time signatures is quick and easy. Conceivably, one could setup Finale to use a 4 line staff and then write everything in C clef (or occasionally in F). I generally count the number of notes needed per phrase/measure in advance. If I'm ever off, changing is an easy fix. I'll have to try the 1/x method. I suppose it's just a matter of what one is used to.

I normally only write out short antiphons and typically don't mess with note spacing--that's the time consuming part. I figure my time would be better spent teaching the choir how to shape the evenly spaced notes on the page than taking the time to engrave it perfectly. Of course preparing something for publication would be another story.