It is possible in theory, but not built into the system. I believe there may be a thread on this, so do a search.
The question is: Is the benefit worth the hassle? How many people really forward that link and is that harming your business? Generally I would say no. I have never done this and no one has ever forwarded me an email link like this.
I did work somewhere where we had a password on some content areas and you'd have to get that password from us somehow. People could have fwd that too I suppose.
Have you enabled forward to a friend? That'd be an easy way to capture information. Josh brings up a good point--it might not be worth the hassle to try and implement something that complex unless you have data proving that this happens frequently and you're missing out on leads.
Wanting to stop someone from skipping the gate is by no means a crazy idea.
The issue is whether you can do this without unduly punishing the original lead. If you send a click-once link, you're saying the lead can't retry (network problems or simply lost the file). That's far too punitive, IMO. If you're willing to let the person download unlimited times from the browser they used to sign up, this is doable using some technology I have in private beta. If you follow me, we can DM about it.
In general, as Josh said, you should remember that if a lead forwards your link, they're helping you out (looking at it optimistically). So the most important thing is to be nice to them. It might be great if you could be stricter with the referral and nice to the referrer, but there are technical obstacles to reliably telling them apart. That said, some kinds of links, like software trials, might be worth the risk and are a good fit for generous, but still enforced, expiration/validity policies.
If someone likes your content enough to share it, and the person receiving it likes your content enough to read it, I think you have already succeeded at content marketing beyond most of your peers !
Sure it would be great to get that person's email as well but I would rather have 5 people read my content and get the awareness and positive mindshare as opposed to having zero or maybe 10% of those people read it plus have their email address.
A good alternative would be to think about how you could make content easy to share (lower friction) while simultaneously capturing the email address of the recipient. Kristen Malkovich mentioned the Forward to a Friend token which is very useful in this regard.
Thanks everyone for your help!