I suspect that many people, like myself, who don't send email
directly from dynamic IPs can't do so because someone they
want to send email to has rejected their mail since it comes from
a dynamic IP address. Technically, there is no reason I couldn't
send email directly. Many probably don't host mailing lists
for the same reason. I can avoid the problem by sending through
the SMTP relay server provided by my dynamic-DNS provider, at
negligible cost and with no need for SMTP authentication (since
he knows my current IP anyway). From my point of view, I just
need to change one item in my SMTP configuration.

A static IP address is another option. That would cost more than
using the SMTP relay server at the provider.

Right now, I still receive email directly. I could channel that
through my dynamic-DNS provider as well, but I lose some control
in the process. Still, I might go this route and let him sort out
or rather tag the spam rather than implementing more anti-spam
measures at my end. (I can adjust the anti-spam level rather
finely along a scale, but not decide on whether to use one method
or another, if I let him do it. If I do it myself, I can also
experiment with various methods.)

While my scheme (everyone using a trusted server to send email
through and rejecting email from elsewhere) would cut out spam
from getting through, your scheme would actually reduce the
volume of spam. Dynamic-IP folks could still send through a
trusted server, as I do now.

It seems to me that your scheme does offer many advantages, but
someone needs to run these magic MX servers. Suppose I opt to
have my provider have the main MX record for me (i.e. route all
incoming mail through him) and try to convince him to run one of
these magic servers. If someone tried to send a lot of spam to
my addresses, or other addresses he handles, then they would get
slowed down. However, a typical spammer probably has a more or
less random list of addresses, meaning they would go through
many MX servers. So, to get off the ground, a lot of folks would
have to start cooperating at once; it can't be built up piece by
piece.

Is there an out-of-the-box, turnkey solution for people who would
be interested in running such a server?

I read that Richard Branson is offering 25 million for someone who
finds a method of disposing of excess carbon dioxide. Why not
convince him (or someone similar) to offer a similar prize (perhaps
somewhat smaller) for coming up with such an anti-spam solution.

Some folks do lose money through spam (due to the time and effort
applied to filtering etc); I think it might not be that difficult
to get enough people together to offer quite a substantial reward
for coming up with a foolproof anti-spam system.