It is not possible to omit the value in custom key=value attributes annotations
According to https://gi.readthedocs.io/en/latest/annotations/giannotations.html#data-annotations, it should be possible to do:
/** * frobnicate: (attributes foo.bar) */
and find an node in the gir. However the annotation is ignored.
To upload designs, you'll need to enable LFS and have admin enable hashed storage. More information