Dear dcb,
Every recent translation project of which I am aware has done a back translation in Paratext. There must be hundreds in existence. I can only guess the reason why you have so few is that others, like me, were not completely clear on what exactly you are looking for and your purpose.
Are you looking for BTs in English only? Is it necessary that the translators used the Paratext interlinearizer, or is a back translation in a separate project sufficient? Are you looking for a particular quality of back translation? Do you need the vernacular translation too? Is a Paratext backup fine, or do you need to be added to the project? How will these be used? Will you be able to protect sensitive work?
If you need access to a large number of back translations, it would seem to me that you’d have better luck getting them directly from the Paratext servers or from SIL’s REAP.
Blessings,
LivingField