Putting a longa as a final note is not a problem: you just calculate how many beats the final bar needs to have in order to accomodate the longa or breve, and then hide the resultant unwanted time signature.
But I sympathize with Thurisaz in wanting to give the player the closest transcription of the original notation. Seventy or so years ago (see Apel, 1941, Donnington, 1963, et al.), the advice was to divide all note durations by 2 or 4; but this severely distorts the look of the music. Nowadays we are smart enough to realise that a breve can be a short note – as its name implies – and do not need to have it represented by a “half-note” in order to sing such music at an appropriate tempo! I have accepted to use, e.g., a dotted semibreve for one that is perfect and divides into three minims. When we do this, it seems legitimate to use the 3/2 signature without the mensural sign before it, as that sign is there to indicate perfection, which is taken care of by the dot after the whole note. But I am aware that this is cheating.
All that said, given that it is a matter of arithmetic and following rules (both of which processes are first nature to computers), it ought to be possible in Dorico to select a situation in which a breve or semibreve is subdivided into three or two of the next lower demonimation, at the command of the user. The appropriate mensural signs could be provided and they would be smart enough to know whether they are perfect or not.
Of course, then the user would demand black and coloured notation, and staves with more than five lines – which would actually be easy to do and would allow us to represent 16C Italian keyboard tablature faithfully – and other things that I have not even thought about… But we do have all the C clefs, which is a major step forward.
The sad thing is that nobody has provided a state of the art means of setting early music, and we may need to stick around for another century before one becomes a reality.