Thanks for the tip.
alt alone didn't work, but alt+space and then "resize" did.
After looking into the beryl settings manager, I noticed that alt+F8 also works to initiate resizing.
The only problem with these methods is that the pointer gets relocated to the center and that I can't therefore pull as strongly as when normally pulling at the top.
I know I can live without it, but it's just part of the fun of Beryl. ^^
(I can also not "throw" windows farther than to the next desktop when using minimal friction.
)