Domain-Specific Modelling Languages (DSML) allow software engineers to use the techniques and tools of Model-Driven Engineering (MDE) to express, represent and analyse a particular domain. By defining DSMLs as UML profiles, i.e. domain-specific extensions of the UML metamodel, development time for DSMLs can be greatly reduced by extending existing UML tools. In this paper, we reflect on our own experience in building rbacUML, a DSML for Role-Based Access Control modelling and analysis, as a plugin for a UML modelling platform. We describe what motivated our choice, and discuss the advantages and drawbacks of using an existing platform to develop a DSML on top of UML and additional analysis tooling.