While recreating the Comet Joint Orient tool over to python I ran into this issue while trying to replicate the world up and tweak vector fields. If you just want to get the value from a float spinbox you can by routing its information into something maya does know how to talk to, usually a line edit, using Qt signals/slots mechanism. However the comet version of the tool has buttons that set the field to a given preset vector and I couldn't get away with only reading the values. The obvious solution to that is to try creating float fields in maya and add them to the Qt interface.
In order to add new elements to the interface we need to know which layout to we want to put things in. For this tool we have two layers of nested layouts in Qt Designer, horizontal layouts for each row of elements and a vertical layout that hold all the horizontal layouts. If we take the Show Axis button for example and ask it what layout its in we would expect something like JointOrient|verticalLayout|horizontalLayout1. Lets see what we get.
The answer to this problem is to organize our interface using widgets instead of layouts, at least in the places where we want to add things after loading the ui file. Once we have a widget we can set it to have a horizontal layout for the things we add.
The ui file can be found here: https://bitbucket.org/jhultgre/maya-scripts/src/b2b5a7c61c7c56a4ed02624e75c2efb5538e8ad0/ui%20files/jointOrient.ui?at=default
Or go to the maya scripts link in the sidebar for all of my maya tools.