🚀 go-pugleaf

You can't find the width until its all been set, because it is not until
then that TeX know how wide each column needs to be.

You might try the following though:

Start up a vbox with the halign in it.

Set a counter to 0 before the \halign.

In the preamble, globally increment the counter each row

Where the paragrap will go, do a noalign of (a) a -ve penalty and (b)
globally set a second counter to the value of the first

Now you have a vbox which contains the halign, so \wd<N> is the width
you want.  Also the 2nd counter gives you the info you need to vsplit
the vbox into the parts of the table before and after your paragraph.
AFter thus spliting box<N>, you can unvbox the two halves with your
paragraph inbetween.

Hope tghis helps
Damian