hide and omit behaviour

[Noeck]

Hi,

I am trying to achieve this for triplets:
The first triplet should show the number, the following not. In rare
cases, I would like to force the triplet number again. I tried to do
this with \omit TupletNumber, but there is one problem as this snippet
shows:

{
  \tupletSpan 4
  \tuplet 3/2 {
    a8-"default" a a
    \omit TupletNumber
    a-"omit" a a
    \undo \omit TupletNumber
    a8-"undo" a a
    \omit TupletNumber
    a-"omit" a a
    \once \undo \omit TupletNumber
    a-"once undo" a a
    a-"why not omitted again?" a a
  }
  r2
}

How can I achieve a "showTupletNumberOnce" like I tried with
\once \undo \omit TupletNumber? Or is the only possibility to do \undo
and then again \omit after wards?

Thanks
Joram