The stylesheets do not currently support such XLink syntax. The current
assembly stylesheet would first need to preserve the xml filenames in
xml:base attributes when assembling resources into a single DocBook
document for processing. That should be done anyway to preserve relative
paths to images and such.
You can use olinks right now. One of the most frequent uses of olinks is
for linking between modular content. Olinks let you avoid hard coding
filenames in your links, which can be troublesome to maintain if you
rename or move files.
Since the idea of assemblies is this isolation (with potential for
re-use?) I'm surprised that olink ideas weren't embedded.
Their indirection has lots of benefits, though I like Bobs later idea
of the processor warning/erroring on missing link targets.