Is it possible to write to a device's clipboard using ADB? I am interested in learning how to write to (and if possible, read from) a rooted Android device's clipboard.

I hope to do so using ADB over USB and I am not interested in installing any apps to help me do this, as this is something I plan on rarely doing.

Any suggestions or help guiding me in the right direction would be greatly appreciated.
Yes, you actually can do this. It's kind of kludgy looking when you inspect the clipboard, but it works just fine.

First off, you can inspect the current clipboard contents with service call clipboard 1 from an adb shell (or, without shelling in first, adb shell service call clipboard 1). It may start out initially blank after a reboot, for example:

# service call clipboard 1
Result: Parcel(
  0x00000000: 00000000 00000001 00000000 00000000 '................'
  0x00000010: 00000000 00000000                   '........        ')
#


You can put text into the clipboard using service call clipboard 2, which basically takes 3 parameters - two ints and the string you want to put on the clipboard:

# service call clipboard 2 i32 1 i32 0 s16 "Hi there"
Result: Parcel(00000000    '....')


To be honest, I'm not sure what the first two parameters are. One answer on Stack Overflow has suggested the first int is "number of items in the parcel" (one in this case) and that the second is the length of the string. However, I've used 0 for the second parameter and it works fine, and I can't find any documentation that matches up with this particular function...so take that for what it's worth.

In any case, it's basically creating a Parcel object with 3 fields, then passing it into the clipboard. The clipboard then unpacks the Parcel and sets the string value passed in as the clipboard's contents. You can see this when you go to retrieve the value afterwards:

# service call clipboard 1
Result: Parcel(
  0x00000000: 00000000 00000001 00000000 00000008 '................'
  0x00000010: 00690048 00740020 00650068 00650072 'H.i. .t.h.e.r.e.'
  0x00000020: 00000000 00000000                   '........        ')
#


Similarly, if you long-press on a text entry field and hit "Paste" after doing this, you will get the text that was set via the call service clipboard 2 line above (and it will look completely normal).

(The above examples come from my HTC EVO, running CyanogenMod 7)