Rule 9.4 addresses maximum jersey size for a goaltender, but it does not specify that a jersey may not have slits (or must otherwise fully encompass the goalie). So I believe you could make a goalie jersey resemble a poncho, provided the fabric itself is to league specifications.
It does say,
"Altered uniforms of any kind, i.e. Velcro inserts, over-sized jerseys,
altered collars, etc., will not be permitted. Any player or goalkeeper
not complying with this rule shall not be permitted to participate in the
game. " 9.1
"No inserts or additions are to be added to the standard players’ jersey as produced by the manufacturer. (Modifications at the
manufacturer are not allowed unless approved in advance by the
League)
No alteration of the neck opening is permitted. " 9.3
"No inserts or additions are to be added to the standard
goalkeeper-cut jersey as produced by the manufacturer. Modifications
at the manufacturer are not allowed unless approved in advance by
the League. " 9.4
" All protective equipment, except gloves,
headgear and goalkeepers’ leg guards must be worn under the
uniform" 9.5
If the NHL allowed what you say....then you wouldn't need a "fat" person. You could just load someone up with massively over-sized padding, and have him plug the goal. The max shirt size prevents that. It would not be that hard for an athletic, fit person to stand in goal to wear enough padding to plug the goal. Certainly easier than getting an obese person to balance on skates.
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