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All of them can at the bare minimum use software packages managed by pkgsrc,
with dependencies and versionning.

Of these three platforms, one of them is pretty much dead: OSF/1 was also known
as Digital Unix and Tru64 and ran on Alpha systems.
After the sale of Digital to Compaq and the sale of Compaq to HP, Alpha systems
were discontinued. According to HP, the last one was sold in 2007.

The official HP Unix is HP/UX, and is currently not supported by LibreOffice.

> Instead of complex advice like "additionally you should install foolib from
> this site, but be careful not to let it overwrite the binary incompatible
> build of foolib from this other site that you might have already"

What exactly are we gaining here by not doing that ?
All live platforms have to be proactive in managing software; if you decide
to freeze some old version of a third-party library or program and include
it in LO, this software will suffer from bit-rot, accumulate uncorrected bugs
and security issues.

You may try to patch it yourself, but this will increase your work and become
unsustainable after a while.
In the end, LibreOffice will become a worse product.

-- 
Francois Tigeot