There is quite a bit of support in SunOS 5.x for running 4.1.x binaries in an emulation mode called "Binary Compatibility" (BCP). This works by dynamically linking the 4.1.x binaries with a shared library that emulates the 4.1.x binary interface on top of 5.x, so there is some overhead.
In Solaris 2.2 and earlier, the programs needed to be fully dynamically linked.
In Solaris 2.3 and 2.4 fully statically linked programs are supported as well. However, they won't obey nsswitch.conf, but use the standard "use NIS if present, fall back to files" approach of SunOS 4.x. Those programs may therefor require a "passwd: compat" line and will only talk to NIS (or NIS+ in emulation mode) or read from files.
Starting Solaris 2.5, mixed mode (partly static/partly dynamic) executables are supported. Whether those programs will use /etc/nsswitch.conf depends on precisely how much was dynamically linked.
Be aware, though, that Sun may drop the binary compatibility package some year. Try to wean yourself and your users from depending on it, even if it means beating on your software vendors to offer "native" Solaris2 applications. But this will happen later, rather than sooner. Sun has not yet announced End-of-Life of this feature.