I don't know what abuse it would be open to. If a leader chooses to give goods to someone they haven't established as trustworthy and going to stay in the guild after whatever is built, that's their own fault. This would also add some use for goods which a guild has ridiculous amounts of excess of, in addition to giving meaning to the observatory, atomium and to a lesser degree, the Arc, for people going it solo, and before you say it, no, the minor advantage of being able to siphon goods from the treasury, in effect, making the three earlier mentioned personal goods GBs and not guild goods GBs is vastly outweighed by the loss of potentially 79 extra sources of goods, all free of charge to trade with.
I agree, this would be a good way for a guild to be even more supportive, and would probably raise the importance of guilds even further (if we think about it, this would mean that a guild could basically decide to directly support of building GBs, completing tasks etc.)
Obviously they would only do that for trustworthy members, which is not really different of what we do now.
For example in my guild we support of raising Traz, and of course all guild related buildings, with both FPs and goods. Now instead of trading tirelessly, leaders could simply send the needed materials to whom they decide.
And if only the leader (and people with the privilege given by the leader) could use the feature, then I don't see any more abuse risk than there it is now.
Oh and of course I'd limit the amount you can give, by age. By that I mean that you couldn't just empty the treasury and give everything to someone. Ie. stone age goods you could give let's say 1000 all together (just an example).
Another restriction could be if you could only give goods from one age at a time, and could only give any more if the before-mentioned is payed back. (though you tell me if that's a good idea)