回答
要求、命令、主張、提案などの動詞に続く that 節には、動詞が「動詞の原形」もしくは「should + 動詞の原形」になります。
She is allowed to see.
誰かがそう命令したという that 節にすると、上の「is」は、その動詞の原型である「be」になります。
Johnsy commanded again that she be allowed to see.
ネットで検索すると、いろいろ詳しい説明が見つかります。
例えば
https://nazebumpo.livedoor.blog/archives/17750572.html
This is not a piece of advice to you I'm talking to myself for my practice of writing skills.
Johnsy commanded again that she'd be allowed to see the last leaf.
Johnsy commanded again that she should be allowed to see the last leaf.
Johnsy commanded again that she would be allowed to see the last leaf.
Johnsy commanded again that she is allowed to see the last leaf.
Johnsy commanded again that she was allowed to see the last leaf.
Johnsy commanded again that she is going to allowed to see the last leaf.
Johnsy commanded again that she was going to allowed to see the last leaf.
Johnsy commanded again that she has been allowed to see the last leaf.
Johnsy commanded again that she had had been allowed to see the last leaf.
Johnsy commanded again that she was allowed to see the last leaf.
I suppose this might be a question that which deformation of 'be' are suitable for?
I can't answer precisely. as a reference only.