Half sustain pedal is possible, but half una corda pedal not?
No, only the sustain pedal allows you to specify a pedal level. You can, however, dress up a sustain pedal line as an una corda instruction by setting the start of the pedal line to use text rather than a symbol and then overriding the text via Properties.
Daniel, thank you for your answer! I tried it as you explained, but it doesn’t work if I have sustain pedal and and una corda pedal at the same time. Half una corda pedal is a common technic, therefore it would be great if we could have this in one of the next updates!
But what effect is half-depressing the left pedal supposed to produce? The “full una corda” on modern pianos actually plays two strings of the trichords, not one. If the hammers were shifted far enough to only strike only one string on a modern overstrung piano, they would also strike a string of the adjacent note.
On a well-used modern instrument, the most likely effect is from half-pressing the UC pedal is some nasty noises as the edges of the hammer grooves hit the strings in an “uncontrolled” manner. Unlike the sustain pedal, the UC pedal really does work best as a simple on/off switch.
Beethoven’s instruction (in Op. 106) “una corda” … " poco a poco due e 'lora tre corde" could be taken literally on contemporary straight-strung pianos when he wrote it, but not on modern instruments.
Rob - shifting the hammers a mm might give you some softer felt. I use this technique on pianos that hasn’t been voiced in a while.
I don’t think I’ve ever seen it in a score, though
I’m with Anders - on a well-used conservatoire piano you want the soft bits of the hammers, not the worn grooves, and that does mean using “half una corda”.
I’ve not seen it notated once in anything but Beethoven. It’s entirely dependent on the piano in front of you, and I always thought it was a useful trick that only a few of us knew about 
I have seen it in the past in several scores, but I cannot remember the composers.
But now I need it for a score for the Dutch publisher Donemus. Also the composer is from the Netherlands.
Yes, it doesn’t work well on every grand piano.
The German pianist Ratko Delorko is writing in his book “Profi-Tipps” (Publisher: Staccato, Düsseldorf 2015) about half una corda (page 145+146).
Well, working round the limitations of the instrument in front of you is one thing, but putting an instruction in the score is something different IMO.
Still, if that’s what the composer wrote, that’s what you have to engrave (unless you can talk him/her out of it!).
Again I have a score with half una corda pedal. I would be very happy if this function would come sometime!
I love the effect of partial una cords on my piano. Never have made it work on any others though.