I am making an assumption here, which is that your idea of openamiga is simply to create a set of programming standards to ensure software is easily portable between various Amiga style OS's.
It's a good idea, and if you can get some quality developers to join your effort, it should be quite successful.
As far a trademarks, copyright etc go, you don't seem to be doing anything wrong. You are simply creating guidelines and community produced code that will ensure that applications created using them are compatible with certain operating systems.
As long as no actual OS code or OS literature is included in each persons final product there can be no legal consequences from Amiga.
As for using the name Amiga in Openamiga - It would be an incredibly tough sell to convince a judge that this was trademark infringement. The only way they could do it is if they could produce a convincing argument stating that by using the name Openamiga, people could in some way be fooled into thinking your product was in some way officially linked with them.
Also you would have an enormous number of precedents to show that what you have done is both normal and legal - for instance 99% of Amiga magazines used the word Amiga in their titles.
Anway IMHO this is a sound idea, good luck with your venture..
