It is certainly possible, but any spacing changes that alter the grid can lead to undesirable results. I tried with a very large and complex orchestral score but it was an editing nightmare!
There is now a really good workaround with MusGlyphs (excellent work by Dan Kreider) in its variants MusGlyphs-Roman/MusGlyphs-Roman Bold: MusGlyphs: Bravura for writing in time signatures and other glyphs - #90 by mati
p.s.: I don’t know if an editor for more complex tempo markings will be present in the next update or even just planned… Mine is just a hope!