ESPNcricinfo understands that Hales has been in talks with MLC franchises about his availability for the tournament, but several teams are worried that the ECB could block NOCs since the new league takes place during the English summer.
While centrally contracted England players appear unlikely to be granted NOCs, players on white-ball-only deals with their respective counties - like Hales - are expected to be given permission to play in the US if they sign contracts.
MLC is due to be played from July 13 to 30 and mainly clashes with the County Championship. The Vitality Blast's Finals Day - which involves only four counties - takes place on July 15, and players involved would miss the start of the competition.
For players without central contracts, NOCs are granted by counties in the first instance, with ECB providing formal sign-off. These have previously been granted to white-ball specialists looking to play overseas leagues during the English season, for both the IPL and the CPL.
The Professional Cricketers' Association (PCA) believes MLC should be treated in the same manner as other leagues that take place during the English summer, "via the principle established in county and ECB central contracts," a spokesperson said. "Ultimately, it is up to the employers' discretion in-season, unless the player is on a specific white-ball version of the standard contract."
Hales' contract with his county, Nottinghamshire, runs until the end of their season in this year's T20 Blast. His only other existing contract within the English game is with Trent Rockets in the Hundred, which starts on August 1 - two days after the MLC final - and if signed, he would return to the UK the day before the Rockets' opening fixture.
Other English white-ball specialists without central contracts who could make themselves available for MLC include Richard Gleeson, Tymal Mills and Will Smeed. At this stage, it appears unlikely that players on all-format deals would request to miss County Championship fixtures in order to play in the US.
Some players are waiting to see whether their counties reach the knockout stages of the Blast, and will only make themselves available for MLC - potentially as replacement signings - if they are not involved in Finals Day.
If the length of MLC seasons expands in future years, as anticipated, some players could make themselves unavailable for some or all of the Blast season in order to sign more lucrative contracts in the US than counties can offer.
Overseas players are signed directly by MLC franchises, rather than through a draft system, and top salaries are believed to be in the region of $150,000 (USD). Quinton de Kock, Aaron Finch, Wanindu Hasaranga, Mitchell Marsh, Anrich Nortje and Marcus Stoinis have all been announced as direct signings.
And MLC has already had a direct impact on the Hundred. ESPNcricinfo understands that Nortje has pulled out of Thursday's draft since his contract in the US is worth more, pro rata, than the most lucrative £125,000 (GBP) deals on offer in the Hundred.