You can use Standard MUI functions/objects to create GUIs, perfectly usable already, the Header Data and library functions for MUI are included.
If you want to use Lazarus/LCL Window like programming someone has to port the LCL-Wrapper to MorphOS/MUI. This is tough work, I tell you. I'm doing this at the moment for AROS http://blog.alb42.de and most of this would also work on MorphOS, there is just a tiny thing in the MorphOS - Freepascal missing, the Resource support. If this is added (most likely by Charlie), I would like to try the LCL also for MorphOS.