While HeinOnline does not support the “at least” function, there is a workaround to use this feature when searching within HeinOnline.
If you search for the same word multiple times with proximity, it will interpret this as searching for the word occurring at least that many times within a distance of X from each other.
For instance, if you search:
“Iroquois Iroquois Iroquois Iroquois Iroquois “~100
this will search for the word Iroquois repeating 5 times within proximity of 100 words.