When redesigning roles or flattening hierarchies, how do you avoid creating ambiguity around accountability? Have you found success with specific role-mapping exercises or RACI frameworks?

Quite an interesting question indeed... I think it all comes down to 2 things: how much time you have, and what is your smallest unit of work.
To avoid ambiguity, the "by the book" approach would be to gather all the information from the people that actually does the job and lay out what are the activities needed to perform the job, then group them by key activities and define key roles from those. Then create teams based on the desired output and make them as autonomous (and accountable) as possible.
Now, that works if you have a lot of time on your hands or a clean slate for this. If not, I'd suggest looking into what is your smallest work unit, this is, is it a person as Individual Contributor? or is it a Team?.... If it is a team, then, as Stephen mentions, the ambiguity and gap should fall into the teams accountability and it should be a team's responsibility to self-organize.
One thing that always helps is to crowdsource the solution precisely with the people involved, or at least the key natural-leaders and thought leaders of the very same organization you are trying to flatten. A flat organization implies a lot of autonomy and a lot of accountability.... start that by making them part of the process

Fascinating question. IMO, best value of role-mapping exercises is to promote open dialogue on who is doing what, etc. However, if it becomes a "playbook" that you expect people to refer to all the time, it will have less usefulness.
Another way to approach this is to build a culture of teamwork and joint accountability. Some ambiguity can be healthy if the culture is one of collectively focusing on delivering results for customers and people. Ideally more people feel accountable for every decision than less. This could also be thought of as the 'owner's mindset'.