Answering my own query: One fast-and-dirty way to do it is with AutoHotKey. I wrote this amateurish script (workaround for the pesky quotes within the command line):
^!F5::
clipboard =
Send, ^c
ClipWait, 2
If ErrorLevel
{
MsgBox, Page number copy to clipboard failed
Return
}
FileDelete, c:\tempcmd.txt
FileAppend, c:\pdfxcview.exe /A "page=, c:\tempcmd.txt
FileAppend, %Clipboard%, c:\tempcmd.txt
FileAppend, " “C:\Dropbox\source.pdf”, c:\tempcmd.txt
FileRead, pdfcommand, c:\tempcmd.txt
Run %pdfcommand%
Return
Now I put the corresponding PDF page numbers into the Scrivener text wherever appropriate, and when I want to call the PDF, I mark the page number and then press Ctrl-Alt-F5: PDF is opened in PDF X-ChangeViewer at the correct page. I can also annotate and highlight the source PDF, as it stays in the Dropbox folder.
Works. Still, a solution within Scrivener would be more elegant!