Until DomCrypt[1] is implemented there is no reliable way to implement a CSPRNG in Javascript (that I am aware of!). The only way to do this securely would be to supply a page that includes some javascript to process a secret key that the user provides to the page which would then generate the CSR. It would be more effective to write all of this sites functionality into a shell script.

[1] http://www.w3.org/TR/WebCryptoAPI/