Is attribute PAYMENTREQUEST_n_CUSTOM supported for PayPal Express Checkout ?

[valley]

Hi there,

we would like to use the same PayPal Business Account for two Rails projects that offer different goods.
We use the Express Checkout way for both projects.
Now to be able to distinguish payments from these two projects, a PayPal guru told me to use the field
PAYMENTREQUEST_n_CUSTOM for sending an identifier that would allow to separate payments.
Since i didn't see any support in activemerchant for this parameter i would like to know how i can use that
param nevertheless and where (setup_purchase, purchase) i would need to specify it so that it will be recognized by PayPal.

Thanks for any hint.
Bests
valley