I think there are a lot of interesting ways to hide part or all of a person during transformations.  I don't know if this is specific to the Belial universe or not so I'll try to make it a bit general.
    The first to mind are transformation machines like your #3 option.  The victim/volunteer walks in, the machine closes around them, the magic happens, and they are released.  This can be partial coverage, such as the machines that create Freyan woman.  Alternatively, they can hide the entire body from view, such as a press.  If you want the transformee to be visible right up to the last minute, you could put them in a glass machine, but have the transformative component applied by a fog or mist that obscures the body.  Indigocarmine's transformations in Belial Idol are a great example of this.  Another idea of his along those lines it to have the transformative agent form a shell or chrysalis about the volunteer, which breaks apart to show the final form.
    If the goal is a "magic show" vibe, you could incorporate different aspects of misdirection that exist in real life.  Put the transformee in a box and have your magician saw him/her in half, only to open the box and reveal two transformed independent halves.  Or put him/her in a box, make it look like they disappeared, then reappeared in a bizarre new form.  Pull an inescapable situation trick.  For example, put the performer in a straightjacket, cover them in chains, then nail them in a coffin.  Normally escape would be impossible, but the transformation turns them into a semi-solid (goop) form than can slide through the cracks.  Or you could modify it like your #2 option, dunking them in a opaque fluid they have to escape from.  Finally, something as simple as having the assistants hold up a curtain could work.
    It doesn't quite fit as well with the "magic show" theme, but you could use something like your #1 example.  If you didn't mind the audience seeing one transformation, you could have them watch a volunteer (or even an audience member) get turned into a beast designed to change other people.  A giant tunnel of flesh with huge mouths, vaginas, or assholes on either end.  A person crawls in one end and is transformed as they pass through, emerging as a freak on the other side.
    If you have some more specifics I might be able to give you some more ideas, but hopefully I've given you a place to start.

While those are great ideas but I'll ditch the Straightjacket part since I prefer all nudity to make the transformation easier.

Although I have some non-beliali magic show transformation ideas since the story I'm thinking is where the transformations are reversible unlike Beliali's though.

Actually I have the same idea for a "Funhouse" idea which these are good ideas while one of them I have in mind is a ballpit where the attendee falls into which is like "Killer Klowns from Outerspace" and the ballpit is 'alive' and slowly envelops and pulls the person sinking to it until submerging in it, which after a while out comes a strange clown like creature (although similarly a clown like creature eats the person which they're "stomach" looks like a circus tent with a ballpit which they slowly sink until submerging which they clown creature opens up like a Russian doll revealing the ballpit being smaller than it looked inside and outcomes a clown creature).

As for the Dark Hallways, I'm thinking of a eyeless creature...

Anyways though I would make all those transformations reversible (since it maybe best if it's non-Beliali) so a person get's to experience all of them (since maybe there can be reversible platform pads around once the attendee is transformed) and of course I imagine all attendees must disrobe nude in this funhouse (to help the transformation process of course) and they know in advance of the transformations it holds. Of course there could be a 'challenge' to make it through the Funhouse entirely unchanged but it's actually a impossible task and they'll might as well enjoy the experience...