I think I've found a simple workaround.
I've added one line to the item's description script editor in the backend
<input class="item_imei" type="text" />
The text filled out in the field ends up appearing in the cart, then again in the paypal checkout with the headding "imei:" (whatever comes after 'class="item_' appears as the headder for that text.
I suppose I could add any type of input provided I include class="item_something"