When creating rthm-seq-palettes you can add empty bars with the requisite number of rests but the easiest thing to do is simply leave them blank:

(5 ((1 ((((3 4) { 7 (28/3) - 28/3 x 6 - })
         ((2 4) (q) g e (e)) ()
         ((5 8)))))))

Note that two empty bars are created there: the first by a simple empty list: (), the second by a change of time signature followed by no rhythms ((5 8))