Looks like this can be done by creating new vm templates with this UEFI setting preset.

Is that something Vates and work on when pushing new templates?

Checked the official docs — short version: there's no supported way to change the OVMF default. It's per-VM only, done inside the guest's UEFI setup, and XCP-ng/XO expose no pool setting, template field, or xe parameter for it.

What the official docs actually say
docs.xcp-ng.org/vms/ has a "Manage screen resolution" section, but it only covers the OS layer:

BIOS VM — set it in the guest OS (Display settings).
UEFI VM — install the XCP-ng Standard VGA Display Driver (part of the Windows Guest Tools package); after that the guest OS controls resolution normally.
The OVMF Platform Configuration menu itself isn't documented there. On the forum, Vates support (Darkbeldin) answered the exact question you're asking with "to my knowledge it's not possible inside the template." That's still the state of things.

Setting it per VM
Device Manager → OVMF Platform Configuration → Change Preferred → pick resolution → F10 to save → Esc back out → Reset (a full reboot; it does not apply to the current boot).

Getting into setup is the annoying part since boot wait is 0s — either spam Esc at the Tianocore logo, or from the GRUB screen press c and type fwsetup.

Making new VMs come up at 1024x768+
The practical workaround, since the setting lives in the VM's UEFI NVRAM (varstored) and NVRAM is part of the VM record:

Build one VM, set the OVMF preferred resolution, shut it down.
Convert it to a template.
VMs cloned from that template inherit the NVRAM, including the resolution.
Caveats: this won't help VMs created from XCP-ng's built-in templates, and there are reports of guest UEFI NVRAM not persisting in some cases, so verify on a clone before rolling it out.

Related knob worth checking first
Separate from the OVMF preference, the emulated adapter caps what modes are available:

xe vm-param-set uuid=$UUID platform:vga=std
xe vm-param-set uuid=$UUID platform:videoram=16
In XO these are the VGA toggle and Video RAM field on the VM's Advanced tab. 16 MiB is the max and is what people use for 1080p and above — with the default low videoram the higher modes may not even be offered in the OVMF menu.

Sources:

Virtual Machines (VMs) | XCP-ng Documentation
Set default resolution for UEFI | XCP-ng forum
VGA on Xen Orchestra | XCP-ng forum
Guest VM UEFI NVRAM not saved / not persistent | XCP-ng forum
How to set screen size for UEFI VMs on XCP-NG | TechOverflow