Actually, the way to go is to do an abstract design. Then you can map abstract semantic units to either level. E.g. [[neg-particle-laa]] can map either to a codepoint in an encoding design or an element in an xml language, according to your taste. Everybody's happy!
OK, that's good. But what would be really useful is in addition to an abstract design, if you implemented your design based on TEI and proposed any additions that you made which were necessary to be added to TEI so everyone can benefit from them. Once it is added to TEI, OSIS can adopt it from TEI since they closely follow each other. This may help facilitate the usage of TEI within Arabic academia.