If I understand you correctly, you would want to turn Show Time Signatures back on in Staff Attributes and then create and apply a Staff Style with Time Signatures suppressed to those measures you want to appear as unmeasured.

Yes, I want some unmeasured time at the very beginning and then switch to measured bars of music for the remainder of the piece. the problem seems to be that the staff attributes are global. At least there's no obvious way to have the staff attribute (don't show time signature) apply to only the first few bars.

Indeed the Staff Attributes are ‘global’.

To change the staff ‘locally’, use a custom Staff Style, and apply that Staff Style to the ‘local’ region.

That is what Staff Styles are for.

To elaborate on J Adrian Verkouteren’s reply:

1) In the Staff Attributes, select the display of Time Signatures.

Now the document displays time signatures everywhere.

2) Create a custom staff style that hides time signatures.

You could e. g. call it ‘Hide Time Signatures’.

Apply your new staff style to the “unmeasured” beginning.

Thank you both. I've managed to get it notated as I wish.