One concrete example for custom comparisons is shown in GeoAlchemy (though it may be overkill for your application). While the examples in the link below use bounding boxes for comparing spatial coordinates, you could use a simpler approach to implement comparison within a tolerance.
See for example the Comparator class which defines "~=" and other operators here:
http://geoalchemy-2.readthedocs.org/en/0.2.4/spatial_operators.html
Since you mention that you were unable to find documentation on how to implement a custom compare function, see here:
http://docs.sqlalchemy.org/en/latest/core/types.html#types-operators
On a related note - if you want to prevent database updates from being triggered if a floating point column remains within some threshold, see the discussion here:
https://groups.google.com/forum/#!msg/sqlalchemy/tCpsGZmjk_w/lOwW93qHV0sJ
(Disclaimer: This is based on a google search, I am new to SQLAlchemy)