With the first official celebration of the national holiday honoring the Rev. Martin Luther King Jr. coming up Jan. 20, there's already a star-packed record to provide musical underpinning. "King Holiday" is a project originated by Dexter Scott King, youngest child of King and Coretta Scott King, executed with the help of rapmaster Kurtis Blow. Dexter King specifically wanted to make a rap-based record because he wanted to reach the audience that has come of age since his father's assassination.

Like "Sun City," "King Holiday" alternates rap verses and sung choruses. The rappers, known as the Holiday Crew, include Blow, Run-D.M.C., Whodini, the Fat Boys and Grandmaster Melle Mel; the King Dream Chorus includes Whitney Houston, local favorite Stacy Lattisaw, New Edition, Greg Phillinganes, James (J.T.) Taylor of Kool and the Gang, El DeBarge, Stephanie Mills, Teena Marie, Menudo and Lisa Lisa and Full Force. The record, which will come in both 7- and 12-inch versions (probably on Polygram) will benefit the Martin Luther King Jr. Center for Direct Non-Violent Action in Atlanta.

Now there's talk of getting everybody together as part of a Jan. 20 NBC television tribute hosted by Stevie Wonder, who was at the forefront of the drive for a national holiday honoring King. The two-hour special will originate from the Kennedy Center, the Civic Center in Atlanta and Radio City Music Hall on a tape-delay basis and will feature music, sports and political celebrities. Quincy Jones is the musical director.

Meanwhile, Cleve Francis, Alexandria's singing cardiologist, has released "Martin," another tribute to King. Francis debuted the song in August at Atlanta's King Center during the 22nd anniversary celebration of the "I Had a Dream" speech. 'Krush Groove' Spinoffs

"Krush Groove," shot for $3 million in 26 days, has grossed more than $14 million in just about the same time and led to more film opportunities for some of its stellar rappers. Run-D.M.C. starts work early next year on "Who Killed Runny Ray?," described as a story about murder on the concert trail. And the Fat Boys have a signed a three-picture deal intended to revive the spirited physical comedy of the Three Stooges. The first picture, still untitled, starts shooting in April and will be directed by Michael Schultz, Hollywood's most active black director ("Krush Groove," "The Last Dragon," "Cooley High").

There's also a "Krush Groove Christmas Party" Dec. 27 at Madison Square Garden with Run-D.M.C., Kurtis Blow, Dr. Jeckel and Mr. Hyde, LL Kool J, Whodini and Washington's own Junkyard Band; it's being coproduced by local promoters G Street Express. A closed-circuit telecast into 100 cities has been postponed until Feb. 15, when the "Krush Groove Valentine Party" holds forth at Philadelphia's Spectrum.

There's also a new line of Run-D.M.C. hats, in two styles -- formal and casual. The formal hats are sturdy Stetsons; the casual versions kangol caps, for "everyday beatboy wear," like sailor hats with the brims turned down.

G Street Express, incidentally, has its own big rapper/go-go/funk concert Dec. 29 at the Capital Centre with Kurtis Blow, Doug E. Fresh, Starpoint, Chuck Brown and the Soul Searchers, EU, Ready for the World and the Junkyard Band. End Notes

Steely Dan, one of the most popular and influential bands of the '70s, is being reformed by Walter Becker and Donald Fagen. They're already in the studio with their old producer Gary Katz. The album will be on Warner Bros., with which they signed after leaving MCA but never delivered an album to before embarking on what was then called a breakup and retroactively termed a hiatus . . .

Which greatest hits were those? Elvis Costello's new greatest hits album on Columbia, "The Best of Elvis Costello and the Attractions," has 16 selections, from 1977's "Allison" to this year's "The Only Flame in Town." However, the CD version has three extra cuts, and a new videocassette compilation includes 22 songs (14 of which have never aired on MTV). Costello's new album, "King of America," is due in January.