Using view helpers in migrations?

I'm writing a bloggish/CMSish thing. I had been using simple_format, but my users requested a rich editor, so I added TinyMCE.

Anyway, I was trying to migrate the existing content (since TinyMCE assumes it's reading HTML-formatted text), and realized that I couldn't call simple_format (or other helper methods) from Migrations.

Can someone enlighten me here?