Yes, this will work, particularly if you make sure to avoid using any features added since 8.5. I would imagine if there are conflicts in user interface features, it my occasionally kill a FMP 11 client. But in general you could access everything. Wasn't 8.5 when FileMaker used a regular app instead of the Java Admin Console? You'll have to make sure it works too.
Note that FM versions 7 to 11 all use the same format, *.fp7. So they can generally be used interchangebly for reading data.
This would not be a stable system you would want to use for a production environment. But I would use this situation to start a conversion.
Using that old of a server can also subject you to security issues, so beware. If the data is important and has value, I would be looking to upgrade it to at least FMS 11. FMS 12 has been out a long time and that is what I would recommend, but you would have to upgrade all of the FMP clients to version 12 to work with FMS 12.