How can you modify a reltio tenant mapping wide via a pipeline parameter?

The reltio tenant is a very important part of the reltio snaps and, so far as I can tell, there is no way to modify it other than manually changing each affected snap.

Is there some method that can be used to simplify that?

For the upcoming release, we will be providing reltio snaps tenant field to get it from pipeline parameters.
For now, there is no such method to do that.

When will the updated release be made public, and is it possible to get early access?

Currently, this is targeted for the 4.12 release cycle, in order to ensure thorough and robust QA and UAT cycles. SnapLogic has a quarterly cadence on releases; however, prior to the release I believe that we have approximately a month of user acceptance testing in the UAT environment. As this is a new SnapPack, we want to focus keenly on quality cycles, and user feedback cycles; the more eyes on the product, the better. Please send me an email at chenry@snaplogic.com and I will introduce you to our Product Management team. They will be able to discuss the road map in detail; moreover, the process to get you up and running with a UAT environment. My apologies for the delay in this message. Looking forward to receiving your email. Have a great weekend!
-Charlie