While I don't know how to change it so that it displays instead of the normal battery meter, that info IS available on all BlackBerry devices, and is very easy to find! Simply follow these commands/selections:

Options (wrench icon) --> Status

On the status screen, you'll see the numerical indicators for signal strength (listed as a negative number representing dBm, whatever that is. i.e. -80 dBm) and battery charge (listed as a %, but it only uses increments of 5%, so it's still an approximate value.) This screen also lists the following additional info:

• File Free: Free space on internal memory, listed as total Bytes free.
• File Total: Total size of onboard memory, in Bytes. For example, my BB 8700g has exactly 64,749,568 Bytes of total onboard memory.
• PIN: Your device's PIN, used for sending PIN messages and IMs via BlackBerry Messenger.
• IMEI: Your BlackBerry's IMEI number, AKA serial number (this may be called something else on CDMA devices, i.e. BBs for Verizon Wireless and Sprint, as CDMA phones don't have an IMEI, they have something similar with a different name.)

This is what you can generally expect to see listed on the Status screen of a GSM BB, but the exact info shown may vary depending on your model of BlackBerry, what version of the OS you have, and what carrier you use. But EVERY BlackBerry I've personally used over the last several years has listed all of the above info, so you have an excellent chance of that info being what you'll see on that screen.

Hope that helps!