Full-back Byrne plies his trade for Clermont Auvergne but that has not stopped Howley selectingBayonne scrum-half Mike Phillips and utility back James Hook, who plays for Perpignan.
The WRU cited the fact that Byrne's domestic season could continue until June 9, the day of Wales' first Test against Australia in Brisbane, as a reason for not selecting the 31-year-old, although he is on stand-by.
Scarlets full-back Liam Williams and prop Rhodri Jones, Ospreys centre Ashley Beck and Cardiff Blues wing Harry Robinson could all make their debuts on tour or in a capped match against the Barbarians on June 2 at the Millennium Stadium.
Openside Sam Warburton will continue as captain after leading the Red Dragons to Grand Slam glory in the spring while fellow flanker Martyn Williams could win his 100th cap against the Barbarians.
The Ospreys contingent in the squad will join a week after the team officially meets up on May 21 as they play Leinster in the Pro 12 final in 12 days' time.
"The Ospreys will have our full backing in their bid for silverware in a fortnight's time so we will not be asking them to join us until a week after everyone else," said Howley.
"They are playing very well at the moment, a win in the final would be a great starting point for our own campaign this summer and the call ups for both Dan (Biggar) and Andrew (Bishop), in particular, reflect the good form the Ospreys are in.
"Elsewhere we have been pretty consistent with selection, it's now down to these players to pick up where we left off in the Six Nations and prove to us that they should be on that plane to Australia."
Backs: A Beck (Ospreys), D Biggar (Ospreys), A Bishop (Ospreys), A Brew (Newport Gwent Dragons), A Cuthbert (Cardiff Blues), J Davies (Scarlets), L Halfpenny (Cardiff Blues), W Harries (Newport Gwent Dragons), J Hook (Perpignan), G North (Scarlets), M Phillips (Bayonne), R Priestland (Scarlets), H Robinson (Cardiff Blues), R Webb (Ospreys), Lloyd Williams (Cardiff Blues), S Williams (Scarlets), Liam Williams (Scarlets)
Forwards: R Bevington (Ospreys), L Charteris (Newport Gwent Dragons), B Davies (Cardiff Blues), I Evans (Ospreys), T Faletau (Newport Gwent Dragons), R Gill (Saracens), R Hibbard (Ospreys), P James (Ospreys), G Jenkins (Cardiff Blues), A Jones (Ospreys), A-W Jones (Ospreys), Rhodri Jones (Scarlets), Ryan Jones (Ospreys), D Lydiate (Newport Gwent Dragons), K Owens (Scarlets), M Rees (Scarlets), A Shingler (Scarlets), J Tipuric (Ospreys), J Turnbull (Scarlets), S Warburton (capt, Cardiff Blues), M Williams (Cardiff Blues)