I agree with you. But I have another two ways:
First:
In Transient model change field type o2m to m2m. Example:
class MyWizard(models.TransientModel):
_name = 'my.best.wizard'
_description = 'Just Best Wizard'
field_ids = fields.Many2many('res.partner', 'my_best_wizard_rel', 'partner_id', 'my_best_wizard_id', string='Partners')
It's the best and simplest solution.
The second is to:
create additional(2nd) Transient model (named like: 'first_transient_model_name.line')
make new field o2m from 1st Transient model to 2nd
In 2nd transient model add your field to your real model with type m2o
...
PROFIT(?)
Look example in: /mail/wizard/mail_resend_wizard.py
But using this method, you lose the ability to directly select the lines of your real Model in the form of a transient model.
You also lose the ability to select several lines of the your real Model from the list at once.
Although, you can make a "button" in transient model Form, which will run modal list-form of your real Model, for allowing selecting several rows you need at once...
It's not an easy way, man.