Readonly radiobuttons: I have defined a series of radiobuttons that replaces an SAP input field:

Radiobutton (0,4) "UB" F[MEPO_TOPLINE-BSART] "UB"
Radiobutton (0,10) "NB" F[MEPO_TOPLINE-BSART] "NB"
Radiobutton (0,16) "YRB" F[MEPO_TOPLINE-BSART] "YRB"

The same screen can be used in a display-only transaction and I would like to change the radiobuttons to "readonly". But according to the documentation there is no "-readonly" option.

If the referenced input field, in your case  F[MEPO_TOPLINE-BSART], is readonly the radiobuttons are automatically readonly and you don't need to take any action.