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In this case where I'm unable to do any sort of 301 is there any other in-page options that might be a reliable way to forward link equity?

The other option is that I keep pressing to change the domain of the login page to a subdomain of the marketing site, which is unlikely at this point, but even in that case the subdomain would cause issues with link equity correct?

The two pages in question here don't contain the same content.  SiteA is a marketing description and features of the product and SiteB would be a login page for that product.  Would the lack of duplicate content cause an issue?

In this situation I have SiteA, and SiteB on completely separate domains.  SiteA is the marketing front for the company and SiteB is an app that company owns.  SiteB receives a fair amount of backlinks as it has the login page of the application where customers link to a branded version for their members to login. Additionally none of that domain is indexable including the login page. SiteB's domain can't be changed to be a subdomain of SiteA as it isn't technically feasible.

Initially I was reluctant to use canonical because as it isn't really duplicate content.  Is there a method for forwarding any link-juice from SiteB to SiteA without the use of a redirect and would canonical be appropriate in this case?  Additionally would SiteB's not being indexed negate any link benefit?

Edit: Typo