Can I do anything useful with these old Android tablets?

Maybe, but the exact model matters more than the Android version. Android tablets use device-specific hardware support, so a generic Linux image usually will not boot just because the processor is ARM. Look up the manufacturer and model, then check whether a maintained mobile Linux project or custom ROM explicitly supports it. If there is no matching device support, expect substantial development work rather than a straightforward install.

Check the processor architecture before spending time on this. Many tablets from that era use 32-bit ARM chips, and current operating systems may not support them or their graphics, touch, Wi-Fi, and power management. Even if a system boots, missing drivers can make it much less useful than the original Android setup.

For basic displays or audio, I would first try to keep the stock system and find apps that still support Android 4.4. You may be able to install compatible APKs manually, but avoid putting these tablets on sensitive accounts or networks. An old Android version is not a safe choice for general web browsing, and apps that depend on Google Play services may still fail.

Before attempting any firmware changes, identify the full model number and look for a recovery image or custom ROM made for that exact device. Unlocking a bootloader and flashing software can permanently disable a tablet if the image is wrong, and the steps vary by manufacturer. With a batch of tablets, test on one unit only and keep the others untouched until you know what works.

Linux is not impossible in the abstract, but I would not count on it as the easy way to revive these. If there is no documented support for the precise model, a small single-purpose use with compatible old Android software is the more realistic experiment. If even that does not work, the screens and batteries may still be useful for parts or recycling.