Hi Danny,
Having read and re-read this, I have two things to say:
1. Not recording the meter
This concerns me greatly as you are not benefiting from the value by entering a meter reading. Do you not generate a PM based on the readings? Do you not analyse the readings to see if there are any changes (trends) that need attention? Are you not concerned that someone has entered the reading and thus that effort has already been expended, but again, you are not benefiting from it?
2. WOMETER
You don't need to have custom fields! There is an object called WOMETER where each reading taken via a work order is recorded in addition to that transaction adding to the METERREADING or MEASUREMENT tables and the ASSETMETER or LOCATIONMETER tables. Relationships already exist, so adding the table to the screen would be simple.
3. ASSETMETER
If you are determined to do the asset's current meter reading, there is a relationship called ASSETMETER which will access the ASSETMETER table, which holds not only all the meters on the asset, but also the last reading.
4. Please note
All the relationships mentioned, list all the meters on the asset rather than a specific meter. If that is your use case is for a single meter then you will need to create a new relationship that focuses on that meter. In Maximo for Transportation it has the concept of a primary meter.