One problem with trying to talk about "easy bass songs" is that every bass player has a different idea of what "easy" is. I've been playing bass (and Maiden basslines) for 19 years now, so most of them are easy for me. But when I try to remember what it was like when I first started, I'd have to say that both Hallowed Be Thy Name and Where Eagles Dare don't qualify as easy for a beginner. Both have some easy sections, but also some intricate licks that can be challenging. The Clairvoyant is easier, but can still be hard for a beginner.
The real trick to playing Steve's parts is simply the issue of right-hand speed. It's not too hard to play fast in short bursts, but sustaining that speed for a long time takes some work. One perfect example is Caught Somewhere In Time, which is a fairly long song with an amazingly fast bass part. When I play that using Steve's technique, I can feel my right hand getting tired by the end of the first chorus. I've found that on a few songs like CSiT, I need to use three right-hand fingers instead of two to keep up.
However, once a player gets over this speed/stamina issue, most Maiden bass parts aren't too hard. Like Wrathchild666 said, E-C-D-E. That progression (or some variation such as E-D-C-E or transposition to another key) accounts for probably 75% of all Maiden songs. But Steve doesn't always stick blindly to those notes; he throws in enough improvisatory fills to keep it interesting. For example, check out the final guitar solo from 22 Acacia Avenue (chord progression: E-D-C-D-E, there it is again!) and listen closely to what Steve plays in the second half of the solo. It focuses on those root notes but also has some variety, including one very-fun-to-play fast descending sequential lick.
I recall that when I first started playing, I found Phantom Of The Opera to be a very challenging song. I was really happy when I finally mastered it; it took something like a month of working on it every day. I'd say Phantom makes a great barometer of where a beginner stands as a player. If a bassist can play that song perfect and up to full speed, they can probably handle any Maiden song.
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