It's not in Canon Law. But, looking at the words, one would have to conclude that there will be some official Church decree with the force of Church Law that guarantees the Tridentine Mass.
To what extent that Mass will be available seems to be what is being "hashed out."
comment from a friend:
A "juridical guarantee" is just a way of saying that something would be guaranteed in law. There would be a process spelled out, binding on all parties, I imagine with the force of Canon Law, that would dramatically reduce the leeway a bishop would have in granting or denying an indult in his see.
Though I have no idea what the cardinal envisions, here are a couple of ideas of how you could have a "juridical guarantee" for the administation of the indult, without having a "universal indult" or granting an apostolic administration:
Plan A
1. Any group of Catholics within a diocese could petition for an indult Mass. Provided they meet certain pre-established, objectively-observable parameters, the bishop would be required to grant the indult. The parameters might include: having a certain number of adult Catholic petitioners; demonstrating that the petitioners are all registered in Latin-Rite Catholic parishes in communion with Rome; having agreement by a priest of the diocese to say the Mass according to the 1962 Missal; having an appropriate church or chapel available for the Mass, with the permission of its pastor; etc.
2. Should the indult be denied, an appeals process to Rome might be established, where the Ecclesia Dei folks would look at the evidence presented by the petitioners, versus the ruling of the bishop, and determine whether or not to uphold the ruling of the bishop, or to grant the indult anyway.
Here's another way the process could be subject to a "juridical guarantee":
Plan B
1. Each bishop could be required to draw up a plan to address the needs of Catholics in his see, regarding indult Masses.
2. These plans would be reviewed by the metropolitan archbishop of his province, and altered as recommended by the metropolitan archbishop.
3. Catholics who felt the plan was insufficient could appeal to the Ecclesia Dei folks in Rome, who could adjust the plan as necessary.
There, off the top of my pointy head, there are two ways you could establish a "looser indult" with "juridical guarantees." I could think of a dozen more (but will spare you). All the "gurantee" means is that there would be a formal process, I imagine based in Canon Law or something equivalent thereto, to obtain the indult, and it wouldn't be left to the whim of each individual ordinary.