It's worth noting that neither the US, Ukraine nor Russia have joined the cluster munition convention. Some other Nato members like Turkey and Greece also did not ban their use.
Still, I don't like the idea to provide such a controversial weapon, but what I like even less is an armed conflict where the protagonist uses different sets of rules.
Also, I'm sure Ukraine may let Nato/close ally monitor planned usage to make sure civilians are out of range.
As for diplomatic repercussions, I think it should not amount to anything game-changer.
The ability for Russia to throw a tantrum on this is weak, but I'm sure they still will and show again how hypocritical they are.
The probability for 3rd party such as China to escalate support to Russia as a result of this is also very weak for the same reasons.
The probability for 3rd party to decrease support for Ukraine is moderate but I think common sense will prevail in the end and it certainly was the focus of US consultation with their ally.
Nato membership might be a little more complex as a result, but as this is out of the question until the conflict is over anyway it does not change much and the urgency of the situation is a priority