I propose to add support for extensions (written in C++) which define custom types. It can be implemented as follows. Extension translates custom type value (for example, fraction) to sequency of bytes in order to store it. Storage engine stores this sequncy of bytes (i.e. BLOB). Extension also supports the presentation of the value in a string form (in order to display it). Also extension can support operations for custom type (for example, arithmetical operations for fractions).
I think that this approach will be better than definition of types using SQL (MariaDB doesn't support it) for nonstandard math types, such as multiprecision floating point numbers, fractions.