cut-away staff for Basso Continuo

You can create a cutaway using (ahem!) the cutaway option in stave/part properties. The cutaway lives on a separate stave. So include a stave just to hold what you want to appear in the cutaway. Then after you have entered notes where you want them, click on the stave and select "Stave/Part properties". If you tick the cutaway box, only measures with notes in them will be shown. You can set the stave size to small in the same dialogue by ticking the (ahem, again) "Small stave" box.

There is a rather sketchy handbook entry here: https://musescore.org/en/handbook/staff-part-properties#common-staff-pr…. I think there is a "How To" somewhere but I can't find it just now.

[Edit] I found the How To, but it predates the implementation of cutaways, so I will not bother to add the link.

Version 3 makes this much easier.

Fill your cutaway measures with all of their notes then right click any measure and choose Staff/Part Properties then:

Set Hide when Empty at the top center to "Always" (MuseScore keeps space for the empty cutaway staves if you don't)
Check cutaway to create a cutaway staff
check Small Staff to make it small

MuseScore should make the spacing work alright. You may need to play with auto placement a bit on the page with the cutaway. On this page, the figured base may cause the invisible staff to move down to avoid it. If you turn off auto placement for these items only the staff should move back up and make the spacing better. If you have both cutaway and figured bass in the same measure, this may complicate things a bit more but we can cross that bridge when we reach it.