Madrid draw has 48 players so it's high level, difficult to get in.
Of course some players are not playing due to injuries but you also have to take into consideration that some spots in the draw are reserved for qualifiers.
However Fabrice is currently ranked 49 and Nico 41 so they should have a chance of being direct entries but I'm afraid the problem is that the entry list is always published weeks in advance so probably they were ranked lower at that time.
In the nineties I was living in Paris and I was always going to the Paris-Bercy qualifications. They were players ranked between 50 and 100 so great, and it was played in a "normal" tennis club so it was very intimate, quite strange to be so close of the players, seeing them everywhere. (now they do it at Bercy though not on the main court)
I think the French boys are just trying to play doubles in every tournament till the Davis Cup, whoever their partner is.
Sebastien has no real intention to play Davis Cup doubles but he is OK to play doubles with Pioline or anybody else just to help them.