I know of no specific citations dealing with any blanket permission/forbiddance of such a thing. So as a direct, simple answer to your question, I believe that it is "allowed" by virtue of not being specifically prohibited. But I'm not sure you're viewing this from the right perspective:
1. Allowed by whom?
"Legality" is a pretty loose concept in WoD. The Camarilla has the Traditions, which are technically mandatory (the Inner Circle might dispatch some enforcers to respond to violations), but in practical terms there is a lot of leeway. Enforcement falls to individual Princes, and, importantly, so does interpretation of the Traditions. They can flex those interpretations as much as they want, up to the point where someone with the muscle to back up their preferences objects and acts against them.
As an example, consider ghouls. Technically, making a mortal into a ghoul is inherently a violation of the Masquerade. And yet, they're pretty common. If a visiting Archon determines that there are too many ghouls, they might respond accordingly: maybe thinning out the population manually, maybe punishing those with "too many" in some way, or laying most of the responsibility on the local Prince and punishing that person in some way.
As another example, consider the Prince of Las Vegas (the name escapes me at the moment). He decreed that Kindred will not use supernatural powers to gamble, and enforces that rule. The same may not be true in another location, like Atlantic City. It doesn't make sense to declare that supernatural gambling is allowed or prohibited in any general sense, as permission is dictated by the whims of whoever is in charge of a given place.
In practice, the limits of what's "allowed" for Kindred are identical to the limits of what you can get away with.
2. What does it mean to be allowed?
The structure of the question seems to imply that, if a Prince were willing to give permission for such an act, it would be sufficiently "allowed" for your purposes. In such a case, the answer can only be "yes". Consider that getting the Prince's permission to sire is essentially a formality, if an important one. If you've earned the good will of a Prince, they might give you permission to Embrace someone whether or not it's a good idea more broadly. If you contrive to get permission through a less-legitimate track (like blackmail), you're still "officially" OK-- it's the granting of permission that counts, not the enthusiasm of the one giving it. That the Prince may have to answer for their decision is secondary.
So if we're using the authority, formal or informal, that broadly governs Kindred activity, our only meaningful guide is going to be the Camarilla's Traditions. Those don't forbid the act, specifically or generally, so it's as "allowed" as anything else that doesn't violate those six rules.
Within the confines of the Camarilla it's formally forbidden for Kindred to kill one another as well (outside of a sire destroying their childe, or a Prince destroying anyone they choose to). But it's far from uncommon, and is clearly part of the standard toolkit of any enterprising vampire that thinks they can get away with it.
Things are less clear with other groups. The Sabbat broadly enshrines an "anything you can survive, you can do" mentality. And they actually do have one or two Abominations among them. Inconnu don't seem to answer to any particular authority, and so are presumably free from any restrictions like this.
3. Why would someone allow or forbid this?
This is probably the best argument for why it might not be allowed to sire a Maeghar. There are various examples of older vampires wanting to severely curtail the introduction of "new" things which might disturb their grip on power, and new, unpredictable powers from new types of entities fall firmly into that category.
But as a counter-example, circumstance also effectively limits what is acceptable to do. I'll challenge your statement that Embracing a Garou is formally illegal in some sense (I'm open to a citation, but am not aware of such a rule offhand). But there are lots of situational elements that might preclude such a thing:
- Garou and Kindred tend to deeply dislike and distrust one another
- Attempts to do so generally fail (though this may not be known to
someone wanting to try it)
- Taking a being that already has enough superpowers to be a serious
threat to nearly any Kindred and likely already despises Kindred,
then giving them even more superpowers, sounds like a plan to
create an uncontrollable super-threat
Together these elements create a situation where permission to Embrace a Garou is very likely to be withheld whether or not there is a formal prohibition against doing so. So even if a Prince has 100% of the necessary discretion to grant a request to Embrace a Garou, it would seem unlikely that they would actually do so. And without that permission, siring at all is strictly forbidden.