I built a custom rom sometime back for uae use (since im not gonna be burning to real chips). All os39 updates applied and all updates since including iconbefast which works quite well. Shoved workbench.lib in the extended rom and its as custom as it gets
. Works well too and can boot any wb without worrying about looking for os39 wb. Still only useful(or not as the case may be) for uae though.
i did try to put in the 040/060 libs , but setpatch didnt seem to like that and just crashed the system.