How do I copy multiple files to macOS's clipboard programmatically?
You can use the AppleScript below to create a bash function that will let you add multiple file objects to the clipboard by supplying their file paths as command-line arguments.  It returns true upon success and false upon failure.

You won't be able to paste the items inside the terminal, but if you navigate to a location in Finder, you can paste the items there.  I hope this is along the lines of what you were after.

use framework "Appkit"
use Finder : application "Finder"

property this : a reference to current application
property NSFileManager : a reference to NSFileManager of this
property NSImage : a reference to NSImage of this
property NSMutableArray : a reference to NSMutableArray of this
property NSPasteboard : a reference to NSPasteboard of this
property NSString : a reference to NSString of this
property NSURL : a reference to NSURL of this

property pb : missing value

on run input
    if input's class = script then set input to ¬
        Finder's selection as alias list

    init()
    clearClipboard()
    addToClipboard(input)
end run


to init()
    set pb to NSPasteboard's generalPasteboard()
end init

to clearClipboard()
    if pb = missing value then init()
    pb's clearContents()
end clearClipboard

to addToClipboard(fs)
    local fs

    set fURLs to NSMutableArray's array()
    set FileManager to NSFileManager's defaultManager()

    repeat with f in fs
        if f's class = alias then set f to f's POSIX path
        set fp to (NSString's stringWithString:f)'s ¬
            stringByStandardizingPath()
        if (FileManager's fileExistsAtPath:fp) then ¬
            (fURLs's addObject:(NSURL's fileURLWithPath:fp))
    end repeat

    if pb = missing value then init()
    pb's writeObjects:fURLs
end addToClipboard


I'd advise saving this script as an .applescript or .scpt file somewhere on your machine using Script Editor.  Then, in your terminal, create your bash function:

pbadd() {
    osascript "/Path/To/Saved AppleScript.scpt" "$@"
}


Then, to use:

pbadd ~/Pictures/*.jpg
pbadd ~/Documents/Some\ file.pdf ~/Music/A\ Random\ Song.mp3