At best an idle curiosity, "Return Engagement," the 90-minute taped drama on NBC tonight, proves otherwise valuable only as a source of amusement for Elizabeth Taylor watchers.Elizabeth Taylor watching is of course a national pastime, and with this pale TV play by James Prideaux, Elizabeth Taylor gets to practice up on being Elizabeth Taylor again.

In the "Hallmark Hall of Fame" production, at 9:30 on Channel 4, Taylor plays - hold on to your chapeaz now - a teacher of ancient history who is trying to hide her past as a vaudevillian from the student body and faculty at a small California college. An eccentric student whom she takes in as a boarder blows her cover and thereby rejuvenates her, almost to the point of having an affair with her.

Almost, but not quite.

Under Joseph Hardy's delicate direction, the story unfolds with a peculiar sense of hesitancy, except for the desperately antsy performance of Joseph Bottoms as the student. In the traditional style of soap opera and of Carol Burnett soap opera parody, people are forever heading for the old coffee cup when the going gets rough, as if perhaps it had been brewed with water straight from Lourdes.

Taylor, her hair styled by no less than MGM vet Sydney Guilaroff, her wardrobe by that ware horse Edith Head, is fascinating to watch mainly if one is fascinated by shadows of former shelves. In this context, some of the lines have a twinkly sort of double meaning. "I don't watch television," says Taylor. "Well, how do you watch old movies?" asks Bottoms. "I don't, says Taylor.

Later he tells her, "You had a great career going," and she says, "I did not have a great career going."

When she intones, "Let momma take you home," to Bottoms, there are echoes of "A Place in the Sun," and when she snaps, "Okay Stu, let's get to work," there is a trace of the crackle of "Virginia Woolf."

The two of them put on a dreadful number at the campus talent show and naturally, if implausibly, the audience in the play goes wild. Well, of coures - It's Elizabeth Taylor almost singing and virtually dancing. Earlier, when faculty members are whispering gossip about her past, she defiantly clomps through a few frowzy hoofer's steps to shut them up.

For the pleasure such moments as that one bring, we can forgive nearly everything else about this trivial wisp of a play.