I'm not aware of such a pass. Do check the websites of MNAC and Palau Guell; in 2015 I think they both had free-access periods. I don't remember whether it was a few hours once a week or once a month. A lot of museums have backed away from doing that recently--either due to crowding/COVID concerns or the need to collect entrance fees from everyone after being closed for some time. Actually, come to think of it, I believe the Picasso Museum may have had a free-access period, too, but that place can be an utter zoo (it was for me, back in 2015); I can't imagine what it's like when it's free (if it ever is these days).
Although Palau Guell is worth seeing, it's less over-the-top/distinctive than Casa Batllo, which isn't on your list. On the other hand, Palau Guell wasn't packed like a sardine can at the time of my visit, so your choice may be a very good one.
Two other worthwhile modernista buildings are the Palau de la Musica Catalana and the Sant Pau modernista site (actually a complex of multiple buildings). You can probably walk right up and buy a ticket to the latter; I've done it twice and no one has reported a different experience here. For the Palau it is prudent to buy the tour ticket a bit ahead of time since the English tours do sometimes sell out.
