Training-My Work Outs Data Screens

is it possible to customise this data field?

You cannot customise that extra data screen you get when executing custom workouts. That means you don't get to change the number of data fields on it, or which data fields appear on it, or in which order or positions.

It shouldn't stop you from using and scrolling to the normally available data screens, however.

What I'm after is for that extra field to show me the total distance, time left, average pace and average HR for the current part of the workout I'm doing. So for example it would show me the data for the 45min section of that run.

You can fit those four data fields onto a single data screen (either Screen‑1 or Screen‑2). Lap Pace and Lap HR will correspond to the third and fourth data items you listed.

Really frustrating (ironic?) that you can't customize the custom workout data screens

This is a bug as far as I'm concerned. Why would Garmin allow customization on all other screens, but not on the customized workout screen?

I know I can scroll to the app-specific screens, but the last thing I want to be doing while working out is futzing with my watch. And the app-specific screens don't tell me where I am in each of my interval circuits, so I have to toggle between to get the information I need.

This is a bug as far as I'm concerned.

You're welcome to form and maintain an opinion of your own, but that doesn't mean it's either objectively correct or subjectively considered to be of merit or relevance to others.

Why would Garmin…

It's just a design decision, and that's the only why that matters – to you as a product end-user and/or customer. Garmin need not justify its design decisions, and it doesn't need to give you room to pretend to argue with it over ‘logic’ that are really just poorly disguised ‘shoulds’ and personal values-based positions.

You want something. You don't have it. You can't make it happen or deliver it to yourself. It's frustrating to fall short and be unable to satisfy one's wants. We get it.

allow customization on all other screens, but not on the customized workout screen?

No, it doesn't. Garmin doesn't allow you to customise the Heart Rate screen (since firmware v4.20), the HR Zone Gauge screen, the Clock screen, or the Back to Start screen. You're allowed to customise the screens (‘Screen‑1’ and ‘Screen‑2’) out of the five normally available ones; that's not even half, let alone all.

I know I can scroll to the app-specific screens, but the last thing I want to be doing while working out is futzing with my watch.

It's your prerogative to decide how you deal with the situation and arrive at the compromise you can best live with, when you're not in a position to change things so that you can have everything without compromise. Even returning the Garmin product because it isn't perfect would still be a compromise, just of a different sort. Getting everything one wants the way one wants it, especially in a single package, is not a consumer right or a customer entitlement buying a product off the shelf.

How's that for the logic of it all, since you seemed to be keen to ask why?