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:

1. create additional(2nd) Transient model (named like: 'first_transient_model_name.line')

2. make new field o2m from 1st Transient model to 2nd

3. In 2nd transient model add your field to your real model with type m2o

4. ...

5. 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.