I’ve gradually started praying the Little Office of the Blessed Virgin Mary. I know I may eventually become tired of praying it. Could I request to make it mandatory from a priest as a penance? This way I would be required to say it.
Permanent penances and trying to bind one’s conscience in such a restrictive way is not a good idea. You can certainly suggest to the priest every so often that he ask you to pray from the Little Office as a one-time penance after that particular confession, but I cannot recommend asking for a permanent penance or attempting to circumvent the priest’s discretion.
If you become tired of reading the Little Office, it is not a good idea to arrange to be “forced” to do so. Just set it aside for a while and come back to it when you are ready. There are plenty of meritorious means by which to honor the Blessed Virgin in prayer.