Recently I discovered that you can instantly respawn if you killbind in the spawn room. (This works on every map) Just killbind, switch class, then switch back to your first class and you'll respawn instantly.
This makes it A LOT easier to A-Pose as Scout, Pyro, Heavy and Sniper. Soo.. yeah just use this trick and you can A-Pose much quicker now.

Before you try any of these, be aware that walking into the resupply cabinet, dying, picking up dead player's items, switching items or taunting will reset your player model and you will stop A-Posing. (as most classes.)
When you have completed the steps for any of these, you will know that you are A-Posing if your viewmodel's (The weapon that you are holding) item is missing (e.g. when you A-Pose as heavy, your minigun will vanish and all you will see is your fist.) Your ammo meter will also either vanish, or remain at 0.

1. Killbind/die while holding the Mad Milk or the Flying Guillotine. (or have another scout killbind/die while holding the Mad Milk/Flying Guillotine.)
2. When you respawn, throw the Mad Milk/Flying Guillotine and DO NOT RELEASE LEFT CLICK.
3. After throwing, pick up the Mad Milk/Flying Guillotine dropped by the scout (or yourself) that died in step 1.
You are now A-Posing as Scout.

1. Equip any of the rage banners. (Buff Banner, The Battalion's Backup, The Concheror)
2. Deal enough damage to fully charge the banner.
3. Then, (make sure you aren't in spawn while doing this) change your banner to The B.A.S.E Jumper.
4. Use the banner's ability, and DO NOT RELEASE LEFT CLICK.
5. While holding Left Click, walk into spawn and touch the Resupply Cabinet.
You are now A-Posing as Soldier.

1. Equip any flamethrower except for The Phlogistinator and killbind/die while holding it. (or have another Pyro die while holding any flamethrower except for The Phlogistinator.)
2. When you respawn, airblast until your ammo is at 0, but keep holding down Right Click, and pick up the flamethrower on the floor.
You are now A-Posing as Pyro.

1. Equip The B.A.S.E. Jumper and any of the Stickybomb Launchers.
2. Make sure you are outside of spawn, and equip any of the shields through your inventory.
3. Walk back into spawn while holding your Stickybomb launcher, and use the Resupply Cabinet.
You are now A-Posing as Demoman.

1. Equip The Huo-Long Heater. (If you don't have it, you can just use any other Minigun, but The Huo-Long Heater makes it easier to A-Pose.)
2. Have another Heavy killbind/die, or use your own Minigun dropped by killbinding.
3. Empty your Minigun and once you reach 0 ammo, keep holding down Left Click and pick up the Minigun on the floor. You are now A-Posing as Heavy.

This process is the exact same as scout's but instead of the Mad Milk, you use the Jarate. (this process will also work with the Self-Aware Beauty Mark, a reskin of the Jarate.)

1. Killbind/die while holding the Jarate. (or have another Sniper die while holding the Jarate.
2. When you respawn, Throw the Jarate and DO NOT RELEASE LEFT CLICK.
3. Pick up the Jarate on the floor while holding Left Click.
You are now A-Posing as Sniper.

i totally didn't spend an entire hour writing this for a bunch of strangers online because i felt like it

I am compiling these 3 classes into one section because they all have the same steps for A-Posing. However, these classes require a friendly Engineer's dispenser to A-Pose.

1. Killbind/die next to the friendly Engineer's dispenser while holding out your Primary weapon. (If you are trying to A-Pose as Engineer, your own dispenser will not work, since an Engineer cannot walk inside his own Dispenser. Dispenser levels will not change anything.) Walk into the Dispenser so that you are clipping into it.

2. While inside the Dispenser, pick up the item that you dropped after killbinding/dying. (and yes, this will also work with other player's items that are near the dispenser, but it has to be a Primary weapon that the player dropped on death.)

After picking up the weapon, you will now be A-Posing. This process is the exact same for Engineer, Medic and Spy.