Valve Chambers for HDPE pipeline

That is a good question (at least if you don't want your valve and adjacent piping bouncing back and forth), and I'm not real surprised you have not gotten any responses. I guess it might be possible to fuse some kind of collar onto the hdpe, but I'm not sure of either the design, nor ultimate efficacy of same compared e.g. to the conventional, fillet welded metal thrust collars on metal pipes to which you referred, particularly in larger diameters (e.g. see apparent contemporary problems noted with electrofusion joining at http://eng-tips.com/viewthread.cfm?qid=229425&page=1 and other references).