A possible solution
The Latex typesetting system has this feature. A solution to your problem would be to use the Asciidoctor-latex extension to use latex to typeset the PDF. However I am not sure how you would instruct Latex to do this using Asciidoctor syntax. I think this would be quite useful to know how to do.
How Latex implements page numbered cross references
It's interesting that the Tex typesetting system is also not a 2 pass system. Latex gets around this by requiring at least two runs of the Tex processor. In the first run it records all the locations of the labels and fills in all page references with "??". In the second run the location of the labels are known from the previous run and so all the page references are filled in with the appropriate page numbers. You may suspect that this may not guarantee correct page number references, and it does not. There are some latex documents whose references numbers never converge to a solution, but apparently these are rarely encountered.
If page numbered references were to be implemented using the Prawn processor, this may be a method to consider.
I usually use the Latex processor instead of the asciidoctor pdf processor because of it's beautiful typesetting. The typesetting system has mostly stood the test of time and this is being improved upon. IMO, the only problem is that the latex markup system is so closely intertwined with the tex typesetting system markup that it is not suitable for producing anything other than statically typeset media. And so we end up with tons of great articles on the internet (e.g. arXiv.org) which cannot be viewed in a web friendly format. I understand there are some good reasons for sticking with pdf only.