I suggest another solution: use the normal installation processes, but install the content in an encrypted way. On postinstall, run some decryption engine that verifies the purchase against the ovi store before unlocking the content. The browser could pass a Session/Purchase ID/MAC that is valid for a few minutes and the store could e.g. retain the IP of the buyer. Only if all data is valid when the unlock call comes shall the content be decrypted.

Easy and cheap to implement, a simple barrier. Not that secure, but needing some work to get hacked - but not economical anymore.

(One problem about the IMEI solution: SIMless devices :-)