Exhibition Fiber is a microporous paper and as you mentioned only a pen will work.
Many who sign works on microporous papers use Pigma pens.
Pigma pens are pigment based and acid free.
More info in the link below.
I learned the hard way to be cautious of pens that are only labeled as "permanent". In some cases, "permanent" refers only to waterfastness, not lightfastness.
+1 for the micron pigma pens. I use them to mark samples in my light fade tests, and the black pigment holds up exceptionally well. Haven't tested other Pigma colors, but then again, for the purposes of signing prints, black ink is a common choice.
Another good pen for this purpose is the Staedtler Lumocolor pen (http://www.staedtler.com/Lumocolor_permanent_universal_pen_gb
, again, I have only experience with the black ink). Whereas the Pigma pens are technical drawing pens which produce a very consistent fine line width, the Staedtler lumocolor is more of a felt tip pen which produces somewhat wider line width and with more line width variation as one applies varying pressure to the tip. This leads to a signature aesthetic somewhat more approaching calligraphy whereas the Pigma pens provide an appearance of precise craftsmanship. Depending on artist's intent, size of print and nature of the art, you may find reasons to have both pen types on hand.
As, Dano pointed out "permanent" marking pens commonly mean water resistant (ie. can go through the laundry without bleeding much), but not necessarily lightfast. What's more disingenuous is when pen makers call their pens "archival" which implies both water fastness and lightfastness yet the ink doesn't deserve such labeling. I've run into numerous pens in that category. The two brands noted above are good safe bets for signing on microporous inkjet media.
Lastly, +1 for the wise approach taken by Phillipe by repurposing some of the black inkjet ink and using it in a refillable pen. This approach does provide a reasonable guarantee that the signature will hold up at least as well as the print under whatever storage and display conditions the print is subjected to over time.