There has a partial effort been made to port rsyslog to Solaris, based on the OpenSolaris platform. In general, it looks quite doable, but a large number of things need to be changes. Some are minor, some require changes to some abstracted functions and some functionality (namely the system logging socket) needs to be completely new implemented.
A port to Solaris (not OpenSolaris) is also possible and we have created an x64 Solaris envrionment and know that we can work in it. In principle, we can also check on rsyslog on sparc, but then someone would need to contribute a machine for this effort.
While we are trying hard to make this feature available as quickly as possible, paid projects so far prevented us from working on the implementation. If you are interested in this feature, you may consider a sponsoring its implementation. That will promote it to the same priority as any paid project and that will obviously make a big difference.
The current estimate is that it will require 8,000 Euros (or equivalent in other currency) to implement that functionality.