Integration with MS Project is probably easiest if you are using Project with a database, rather than individual project files. In the former case, you would use the standard DataExchange techniques. You could also use sd_event, I suppose. You could either push or pull data, depending on your requirements. Just make sure that each side knows how to uniquely identify objects. In the latter case, it might be quite complicated, but VBS might be useful.
For Rational, I presume you want to exchange data about CIs. Again, use DataExchange. However, in my experience the granularity of data in Rational is far too detailed for the CMDB.
For ARS, you can again exchange data between its database and OVSD using Data Exchange. I have never tried using ARS workflow with OVSD, which sounds like an idea so convoluted that it might even be bad.
If you supply some more detail about what you want to do, we could get more specific.
The difficulty of integrating other tools with HPSD is that you have to speak the same language (our your integration has to translate it) about: - CI's - Process - Status - Support Groups - ... When you have these parameters under control...you can integrate everithing you want!
I think that integration with ARS and Ration is very simple. I beleave that with sd_event is possible. But i don't know the MS Project architecture, so i can't see the possibility of the integration. We want to integrate MS Project with Project item in Service Desk, first the date and status.