I don't have a definitive answer because I specialize in WMQ exclusively. However, I believe the answer is "no" for the most part. (More on that in a minute.)
Regarding WMQ IBM makes available exit points to tailor the behavior of the channels, API calls and authorizations. Exits are very well documented and perform narrow functions within the scope of a particular action - i.e. receive a message, initiate a connection, etc. These are written in C and, more recently, Java. For the most part these are unused and customers I talk to generally cite complexity. They want something customizable through configuration and not through low-level code. I suspect other MOM vendors experience similar requirements from customers.
What does this have to do with your question? My take on this is that if customers are reluctant to code up exits with limited function, it seems far fetched that they would code up a full-featured and robust client that supports reliable message delivery, one- and two-phase commit, client-side exits, diagnostics, and all the other functionality that WMQ channels provide.
Assuming that this task was undertaken by an open-source team capable of that level of code, who would support it? the MOM vendors currently provide end-to-end support when using their proprietary clients. The notion of how a trouble ticket might be resolved when using a third-party client that is community-supported is a bit scary to many customer. For example, IBM supplies add-ons for WMQ called SupportPacs. Although there are SupportPacs that are fully supported and are considered product extensions, some of the SupportPacs are provided as-is. Many of my customers won't run as-is code even when it is supplied by the vendor.
Finally, there is the notion of the interface contract. WMQ supports a few verbs with a lot of options. The underlying channel protocol is MUCH more complex. When WMQ v7 came out, the channels had considerable new functionality and tuning. this was possible at this scale because the internals are not exposed to clients and so IBM was able to make massive changes without fear of negative impact to 3rd party clients. Exposing all of that would create dependencies on an order or two higher magnitude than exist with just the API's exposed.
So, according to my theory (I don't pretend to speak for the MQ development team here) the big MOM vendors have a vested interest in not exposing their channel protocols to independent developers. The new wrinkle here is AMQP which I alluded to above. It defines the wire protocol and allows each vendor to code a compliant product. Although this provides the opportunity you describe for open-source solutions, the ability of any one implementation to improve the product is limited by the fact that they don't own the protocol. For the time being though I don't expect you'll find any of the big MOM vendors exposing their wire protocols for 3rd party development. That said, this is just a guess and if I'm wrong, I'm sure someone here will jump in and provide the counter-example.