Is it possible to use different (early music?) symbols for quarter notes?

See https://musescore.org/en/handbook/noteheads-0

What about starting the score in 3/4, right-clicking on the time signature and changing its appearance to 3/2 and then entering the notes as quarters, dotted quarters and eighths and then (at the end) selecting all notes and changing their heads to half-note (minim) type? That would get you your tails etc. but you would, of course, really need to know ordinary music notation as well as the archaic notation to avoid mistakes.

Well, this is a rather well known question when working with musica mensurata, for which there are already generally excepted solutions. For a proper rendering, I believe it is important you understand the context.

What you are looking at is an example of coloratio. In coloratio, the colour of the note is changed to express a change in either tempus or prolatio from the mensural sign given at the beginning.

To put it simply (and to skip a number of false starts, transitional periods and idiosyncrasies), the system originated with the mensurabilis musica of the Ars Nova and with black notation; in it, a tempus or prolatio alteration was mainly expressed by using red ink instead of black (whence the name): under a tempus imperfectus sign (1 brevis = 2 semibreves), when 3 semibreves should fit 1 brevis (we now would call it a triplet), the semibreves and smaller notes in which a semibrevis could be divided were inked in red. Same thing for fitting 2 semibreves to a brevis under a tempus perfectus and/or for prolatio (semibrevis to minima ratio).

With the white notation, the main mean was to exchange the filled / hollow status of the note head. This raised the problem of distinguishing between a coloured (filled) minima and a seminima or vice versa between a coloured (hollow) seminima and a minima. The first case was usually left unaddressed, as usually it is rather easy to understand what is going on. The second case was often (but not always) addressed by adding a hook to the coloured (hollow) seminima to make it clear that it is not a minima.

This worked rather well until, approximately, the end of XVI century; at that time the progressive replacement of mensural signs with our time signatures, left for a while the system in disarray with a transitional period in which (simplifying...) each author or local school did whatever he/they see fit.

The generally accepted solution, for all these cases, is to render the notes with their nominal value and mark the span of notes coloured in the source with a pair of opening/closing square half-brackets: ┌ ┐ . You may find some examples of both a MuseScore score and the final intended result in this score of mines .

So, what you are looking at is definitely a coloured seminima and should be rendered with a seminima. What to do about it being coloured depends on your philological concerns and on the intended goal/audience of the score you are preparing:

A) If the score is just for direct performing by you and/or your group, you may easily simply ignore it.

B) If the score is intended to be (or to become) a critical edition, you should note the fact in some way. Either by marking the coloured passages with the half-brackets (which however is more fit for longer passages, here we just have single notes), or by a sentence in the critical notes addressing all these cases at once.

A few notes:

1) Note that the 3/2 at the beginning is most likely not a time signature at all (EDIT: or at least, not entirely!): it usually indicates that the tempo is 1.5 times faster (3 / 2 = 1.5) than the previous piece, while the mensural sign of that previous piece might still be in effect (but in this transitional period, it may depend on the context).

2) In this particular case, the coloratio is largely useless, which may support ignoring it in the score proper and adding just a collateral explanation in the front matters.

3) This specific notation kind of became a locus communis and might be found as late as Couperin (for instance in the 12eme Concert Royal).

4) Any attempt to render the original shape of the sign is likely to be rejected by the musicologist as a purely antiquarian affectation: the meaning of the sign is absolutely clear and there is no need to puzzle the performer with typographic eccentricities.

Hoping it might be of some use,

M.