Custom line-break requirement

I've got an interesting problem that I've been struggling to find an answer
to.

I need to produce some text in the form of an address block, which only line
breaks on ',' characters not on ' ' characters.

I.e. Currently I get:
1234 Thingy drive, New York, New
York, Some county, AC 0DE

Where I want address elements to stay together:
1234 Thingy drive, New York,
New York, Some county,
AC 0DE

I've tried several techniques to no avail:
keep-together attribute doesn't seem to have a way to override the logic to
do this.
Inserted a white printable char for each space, still gets treated as a
place to break.
Can't reliably calculate line width of manual breaking with a variable width
font by char count.

Any ideas?

Thanks, JV
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