Writer: will I ever be able to define conditions using internal variables?

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Instead of fields, create a different page style for the last page. Then enter whatever is needed in the footer.

Headers and footers are the same across all pages with that style. So a condition like currentpage == page is not possible in the footer. Note that page confusingly refers to the total number of pages.

Furthermore, as you apparently know, there is no such variablecurrentpage. According to https://help.libreoffice.org/Writer/D...:

You cannot use internal variables, such as page and chapter numbers, in condition expression.

To me, this seems unlikely to change and would require a great deal of effort to implement. However, you could file an enhancement request and see if anyone is willing to tackle it.

A workaround to determine in the text body whether it is the last page is to increment a variable on each page. I do not see how this would be useful, but here is a sample file: Last Page condition.odt

You can do quite a lot already. See, for example, @JimK's useful sample for a conditional counter in an earlier Q&A

See also the help information on "Defining Conditions " and "Variables", and the internal links in those pages. It might well be possible to accomplish your scenario, though I haven't attempted that one myself.