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There is no inband negotiation like there is with 802.3z or SGMII,
so this makes no sense.

Oh, that's what I feared. I read some docs but probably will need more
:)

Anyway, the reason to use in-band negotiation was also to avoid using
fixed-link. It would work but always report the link is up, which for
the user isn't a great experience as we have a way to detect this.

What would you suggest to achieve this in a reasonable way?

The intention of this test in phylink_of_phy_connect() is to avoid failing
when there is no requirement for a PHY to be present (such as a fixed link, or
an 802.3z link.)  However, with 10G PHYs such as the 3310, we need the PHY
so we can read the speed from it, and so know whether to downgrade the
MAC to SGMII mode, or having downgraded the MAC, upgrade it back to 10G
mode when the PHY switches to 10G.

I'm guessing that you're wanting this for the DB boards, but I don't see why.
Do they not have PHYs?

New Solid Run board MACCHIATObin Single Shot doesn't has  3310 PHY either, like DB boards.
https://www.cnx-software.com/2017/12/20/solidrun-macchiatobin-single-shot-networking-board-launched-for-269-and-up/

Stefan,
Best Regards.