Glenn:
I am using 100% epoxy material to fill vias 100%.  This material is not
conductive but does a good job.  You can use screen printing method
(pushing through the via holes).

For conductive material, you can use either silver, copper or a
combination in paste form using the same method. I did not use this
method but the manufacturer claims: either paste can fill vias 100%.
If you want the manufacturer and/or a contact person, write me an
e-mail.
Hope this helps.

Dejen Eshete
Santa Clara