If you want to display numbers (including the number 1) on full bar rests, but only if all voices have rests simultaneously, here is a "negative" engraver (or "un-engraver").
The un-engraver removes the rest number grob if there is any note head between the bar lines. To avoid problems with cue notes inside a full measure or multi-measure rest, only noteheads with a minimum size trigger the removal of the full-bar rest numbers.
This display method is often preferred if you display only a sub-score of a piece, for example, a choir score.