Using only one public key; no way to sign it

[Clint (cpctc)]

I want to use gpg with only a single public key.  I know the key is correct
and trusted and not compromised, but I can't find a way to tell gpg to trust
the key.

I don't have a private key to sign the public key with, and I don't think I
should need one.

I've found some relevent posts:
http://www.roads.lut.ac.uk/lists/g10/1998/05/0021.html
http://www.roads.lut.ac.uk/lists/g10/1998/05/0022.html

but nothing since then that's applicable.

Basically, I want to do the following, starting from a fresh installation of
gpg:

gpg --import mypubkey.pgp
gpg --encrypt --recip "mypub" hello.txt

.. without any user prompts.  The closest I've come is:
--
[c:\temp\gpg]gpg --encrypt --recip "mypub" --armor --quiet --batch --yes
hello.txt
gpg: abcd1234: no info to calculate a trust probability
gpg: no valid addressees
gpg: hello.txt: encryption failed: no such user id
--

If I take out the "--batch", I get a prompt:
--
Could not find a valid trust path to the key.  Let's see whether we
can assign some missing owner trust values.

No path leading to one of our keys found.

<key info>

It is NOT certain that the key belongs to its owner.
If you *really* know what you are doing, you may answer
the next question with yes

Use this key anyway?
--
I tried "--trusted-key", but it said:
gpg: key abcd1234: no public key for trusted key - skipped

so I don't understand what it's good for.

I'd like the "--strong-yes" or a "--force-trust-key", or a workaround that
does not require any console input... commands that can all be run
with --no-tty.

Thanks.