Re: Has anyone tried a test in a LDOM on some modern SPARC server?

I haven't but there is NetBSD src/sys/arch/sparc64/doc/TODO which I
think tracks the latest status.

If you are mostly interested in testing user space applications for
NetBSD, then systems based on sun4u may be the only choice for now.
This is what I use with my own applications, i.e. I use NetBSD
cross-compiler on x86-64 to produce sparc64 binaries and then test them
on Sun Ultra10. Building even modestly sized executable on this machine
is an exercise in frustration, mainly due to the slow 440 MHZ UltraSPARC
CPU.

It was possible to cross-build NetBSD on SPARC Solaris years ago, but
last time I looked it had several issues resulting in build failures
(both in src and xsrc trees). I did submit bug reports, but not sure
there was much interest in fixing those, so they may still persist to
this day. Once those issues are resolved, then you could use SPARC
Solaris for all cross-building (various SPARC T2 to M8 CPUs are quite
suitable for this job, provided you feed them with enough concurrent
tasks to keep all CPU strands busy):

1. Build NetBSD cross-compiler tools + release binaries.
2. Use cross-compiler tools to build your software.
3. Transfer binaries to sun4u system and test there.

If in step 2 you use GNU software with autotools, then it may not build
properly, as some of it is not designed to work with cross-compilers.

For my own software I use custom Makefiles and scripts, which are
designed to be portable across multiple tools chains: native and
cross-compilers, GCC, Clang, Sun Studio, etc. I think I looked at
using autotools many years ago and quickly abandoned them.