Hello Tom Wrigglesworth,
Welcome to microsoft Q&A, Thankyou for posting your query here.
Yes, it is possible to install Apache2 on all the Ubuntu instances in a scale set and add a custom page to them.
Scale sets are designed to allow you to easily manage, deploy, and scale out hundreds or even thousands of virtual machines in a synchronized manner.
please refer this documentation for the same.
or else you can use custom image
https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/tutorial-use-custom-image-cli
Hope this helps you,
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