From the long summary you might guess this description could be a little complex - I'll try and keep it simple...

Running:
squeezeboxserver-7.5.3-1.noarch
and
squeezeslave-lnx26 from squeezeslave-1.0-210-lnx26.tar.gz with "/usr/local/bin/squeezeslave --volume off --retry --daemonize /var/log/squeezeslave.log"
on
kernel
2.6.33.7-149.rt30.1.fc14.ccrma.i686.rtPAE

Radio
Firmware: 7.5.3-r9283

Boom
Firmware: 52

Playing all the above in sync from My Music local music library works almost flawlessly - almost because every so often sounds stops coming from the Radio although it indicates playing, momentarily pausing and un-pausing starts the sound again.  (Strangely, sound starts for a brief moment on touching the pause button, then of course stops because it's paused, and starts again on touching the pause button again because it's now playing - this seems like a Radio bug and not the subject of this bug report.)  OK, sync is excellent on My Music.

BUT, squeezeslave is unable to keep sync or even work when playing all the above in sync from Internet Radio.  The symptoms vary:
* when attempting to play mms://streaming.radionz.co.nz/national-mbr the whole lot lock up and won't even buffer;
* when playing http://opml.radiotime.com/Tune.ashx?id=s50646&formats=aac,ogg,mp3,wma,wmvoice,real&partnerId=16&serial=fe3391ce6a5ebcb346bd2a125aeb6c8d (BBC WorldService via RadioTime) squeezeslave runs (it seems) at 2 x speed pitched up appropriately, both Radio and Boom keep sync;
* when playing rtmp://wsliveflash.bbc.co.uk:1935?icon=aHR0cDovL3d3dy5iYmMuY28udWsvaXBsYXllci9pbWcvcmFkaW8vYmJjX3dvcmxkX3NlcnZpY2UuZ2lm&livetxt=cHVic3ViLmxpdmV0ZXh0Lndvcmxkc2VydmljZQ==&ttl=MTI5NjQyNzA0OA==&tcurl=cnRtcDovL3dzbGl2ZWZsYXNoLmJiYy5jby51azoxOTM1L2xpdmU/X2Zjc192aG9zdD13c2xpdmVmbGFzaC5iYmMuY28udWsmYXV0aD1kYUVjdGFVZEdhd2R6Y09hS2IuZDhjQWJHY09icWJvZHhkcy1ibnJFR0QtYldHLUtuckdDbnlxS0RuRXR4RSZhaWZwPXYwMDEmc2xpc3Q9ZW5ldWtfbGl2ZUA2NTEy&url=aHR0cDovL3d3dy5iYmMuY28udWsvbWVkaWFzZWxlY3Rvci80L210aXMvc3RyZWFtL2JiY193b3JsZF9zZXJ2aWNl&app=bGl2ZT9fZmNzX3Zob3N0PXdzbGl2ZWZsYXNoLmJiYy5jby51ayZhdXRoPWRhRWN0YVVkR2F3ZHpjT2FLYi5kOGNBYkdjT2JxYm9keGRzLWJuckVHRC1iV0ctS25yR0NueXFLRG5FdHhFJmFpZnA9djAwMSZzbGlzdD1lbmV1a19saXZlQDY1MTI=&br=MTI4&subscribe=ZW5ldWtfbGl2ZUA2NTEy&swfurl=&live=MQ==&streamname=ZW5ldWtfbGl2ZUA2NTEyP2F1dGg9ZGFFY3RhVWRHYXdkemNPYUtiLmQ4Y0FiR2NPYnFib2R4ZHMtYm5yRUdELWJXRy1LbnJHQ255cUtEbkV0eEUmYWlmcD12MDAxJnNsaXN0PWVuZXVrX2xpdmVANjUxMiZhaWZwPXYwMDE=&update=UGx1Z2luczo6QkJDaVBsYXllcjo6QkJDTVNQYXJzZXI=&.aac (BBC WorldService via BBC iPlayer) squeeze slave runs (it seems) at 1/2 speed and pitched down appropriately
* Weird!

I filed this bug under SBS because it seems most likely the culprit is somewhere within SBS's Internet Radio function - perhaps not processing streams and unable to keep up somehow.  Else, why would playing from My Music work so well?  This is so frustrating because this set up is so tantalisingly close to being perfect!

All questions and answers much appreciated.
Many thanks for such a wonderful thing as this whole project!