EDIT (only this line): Just discovered that you can copy and paste from the W3C page. E.g.: 12/8 is U+F605.
This may do the trick with single digit numbers. I still haven’t figured out how to write 12/8 correctly. I am working with Liberation Serif and Bravura Text fonts.
-
a
: the text before the time signature
-
U+F5D0
Alt+X
: the lower 4
-
U+F5D9
Alt+X
: the upper 8
-
U+2008
Alt+X
, U+2008
Alt+X
: two additional spaces (maybe another space code can be useful)
-
a
: the text after the time signature
- select both 4 and 8, and set Bravura Text

Unicode codes:
U+F5C8 for lower 0 U+F5C9 for upper 0
U+F5CA for lower 1 U+F5CB for upper 1
U+F5CC for lower 2 U+F5CD for upper 2
U+F5CE for lower 3 U+F5CF for upper 3
U+F5D0 for lower 4 U+F5D1 for upper 4
U+F5D2 for lower 5 U+F5D3 for upper 5
U+F5D4 for lower 6 U+F5D5 for upper 6
U+F5D6 for lower 7 U+F5D7 for upper 7
U+F5D8 for lower 8 U+F5D9 for upper 8
U+F5DA for lower 9 U+F5DB for upper 9
Tested with LibreOffice 6.4.7.2 (x86); OS: Windows 6.1.
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