SAP are releasing a limited-scope HTML 5 editor later this year, initially restricted to editing BPMN models. I haven't seen it myself, apart from a few screen shots, but it's very likely that it'll give you the features you need. I suggest you raise a support case with SAP, asking for a preview.
It may be better investing your effort in talking to SAP than doing your own customisation. Nobody likes spending time inventing something that gets replaced almost immediately 
.