I completed a new srv resolver plugin for sipwitch and a new resolver plugin api to support it. We used a plugin because we are using libruli for srv lookups, and it has a lot of dependencies of it's own, so I wished to make those all optional for sipwitch. There are many specialized use cases where sipwitch may be deployed where it never needs to do an exteral lookup, and so there is no need to carry all that baggage in the default server. This is already true for sipwitch zeroconf support, which is a plugin so that the default server image need not depend on avahi client libs even when zeroconf is not being actively used.
The new sipwitch srv resolver plugin is being released today as part of sipwitch 1.7.1. In fact both antisipate and sipwitch will both support srv lookup now, and both can also successfully find each other with zeroconf on local subnets, which is part of our goal for auto-configuring sip services.
In the case of antisipate, zeroconf and srv lookup are expected to be normal or default services, and anyway it is likely to be deployed on desktop machines rather than also supporting deeply embedded routers, so having them built-in to the client is not as big a deal. However, we may look at going to plugins in the new client too in the future...