AIUI there is no rule. However, non-US players signed to ECHL contracts may be limited by H1-B visa availability (excluding Victoria).
(Players on NHL contracts are in a different visa pool.)
This is the same pool of workers that are the "skilled" workers hired for tech firms, etc.
You'd be correct LS. Not sure when the rule was pulled but when I followed the Columbia Inferno during the lockout, there was no mention - ever - of an American-born player rule and I'm fairly certain that most of the time we had fewer than 6 Americans on the roster. Also correct about the visa availability issue, although it's for H-2B visas, not H-1B. After Eric Boulton signed with us, he had to sit out a few games while he got an H-2B visa since, having been an NHLer, he had an H-1B visa instead.
If you dig deep enough in the dailypress, there's a lot of info about salary cap. Stuff like weekly cap values, the american player rule, franchise fees and how they gradually increased.
If I get enough time, and I can probably pull it off at work since we're been sooooo slow, I can probably start a thread about franchise entry fees, franchise salary values, player pay, etc. throughout the years.