I have a model representing a tree with 2 columns, one for item name, one for state (enabled/disabled). I want to set an accessible role to checkbox because I think like this a screen reader can be able to say if an item is enabled or disabled. Currently I use the accessible text role to do that but it's not a good solution for me.

i haven't used the accessible framework for a while, but i think you will have to create a custom accessible interface (plugin or in source) for your tree widget to represent the checkbox in the interface

Qt::AccessibleTextRole

Why not return the values you want for that role ?

    case Qt::DisplayRole :
        if(orientation == Qt::Horizontal)
        {
            switch(section)
            {
            case COLUMN_NAME: return tr("Event");
            case COLUMN_CHECK: return tr("Enabled");
            }
        }
        break;

    case Qt::DisplayRole :
        if (index.column() == COLUMN_CHECK)
            return (m_ttsselected & m_ttsevents[index.row()])? tr("Enabled") : tr("Disabled");
        Q_ASSERT(index.column() == COLUMN_NAME);
        switch(m_ttsevents[index.row()])
        {
        case TTS_USER_LOGGEDIN :
            return tr("User logged in");
        case TTS_USER_LOGGEDOUT :
            return tr("User logged out");
//...

one for state (enabled/disabled)

How is this represented? QTreeWidget can represent "chackboxes" natively