If I'm getting your drift, these shots are for effect/design/colour -
in other words, they are more about being decorations/decor than
photographs.  I'm not putting them down, by the way - my point is that
the viewer is not going to feel a strong force pulling them up close to
view the print.

Well, not as close as they might do if it was an aerial photograph
('oooh! is that my house?, I think I can see my dog in the back yard,
have you got a magnifying glass...?')  or a landscape with lots of
small and 'individually interesting' details - like people fishing on a
jetty, or sailboats with details on the sails, etc.

So given that psychological rather than technical assessment, I would
say.. go for it!  100 ppi works out to about 30" x 20", and would
probably be fine, so might 50 ppi - 5 feet wide?  In all seriousness, I
think these are the type of images where making them bigger will simply
invite your viewer to step *back* further, and therefore the sky is the
limit.

Just my 2c..  Oh, and use a good interpolation program, eg QImage..
Pixellation *isn't* cool.  (O:

Look at the existing framed art.

If the client was reasonably satisfied with the previous layout, I'd use
that as a guide in sizing the prints; size them so they can be framed to
the same size as the existing art.

Another possability is get the prints done by a pro lab. I've had 6mp
files printed to 11x14 by a lab using a Lightjet with no problems about
sharpness. Wasn't even pushing the capabilities.