There's already a UA invocation that allows the Warlock to wear heavy armor, why not just use this if you want better AC than Mage Armor?
UA may not be allowed. I personally don't allow that particular Invocation.
Allowing homebrew but not UA would be pretty weird though. Also why don't you allow that particular invocation? It's honestly worse than just multiclassing.
A couple things about that UA Invocation:
1. It is only usable by Pact of the Blade Warlocks, and the vast majority of Pact of the Blade Warlocks are Hexblades, who would not even need this Invocation.
2. Being Proficient with any set of Heavy Armour would not negate the speed penalty due to lack of strength to move in that armour, and I have seen very very few Warlock setups that have a high STR, High CHR combo.
That UA undead warlock would make a pretty good melee alternative for a Hexblade and could make fantastic use of that invocation. You could start at 16 str and 16 charisma, get great weapon master as your variant human feat and focus on your strength. Wont need to touch charisma until level 12 when you can grab lifedrinker.
Just because you don't make CHA your priority as Bladelock it doesn't mean you won't be casting spells. There are plenty spells that don't require high CHA to work just fine. If you don't even know that it's plenty clear that you never really looked closely at your spell list or never played a gish character. The only reason Hexblade characters focus on CHA that much is because Hexblade is somewhat broken in multiple ways, one of those being able to use CHA for their weapon attacks. If your aim is to min-max though ... just play as Hexblade. It's without question the strongest patron even if you play a traditional Eldritch Blast slinging Warlock.
I'm playing a Pact of the Chain Celestial Warlock with just 14 Charisma, and I'm having a great time. I usually play gishes, and I'm really enjoying this character. Getting to upcast Invisibility and Fly and get the spell slot back on a short rest is fantastic. Whenever we want to do party wide invisibility or party wide flying, the Bard and Wizard can each use a low level spell slot, and I upcast it and get the Fighter and Barbarian covered. With my 21 AC, it's a blast using Shadow of Moil, it makes me a real pain to damage, especially since I can ask the DM "does that spell require you to see your target?" whenever he tries casting a spell on me when I have that up. Shadow Blade was fantastic before I got a Flame Tongue Shortsword. Dimension Door has turned out to be quite useful because the Bard and Wizard both have it, so because I picked it up as well, we can use it party wide as well. And because there's no Cleric in the party, I also picked up Cure Wounds, Lesser Restoration, Revivify, Dispel Magic, Remove Curse, and Greater Restoration. And for my first Mystic Arcanum (which I chose last week), my DM let me take the UA spell [Tooltip Not Found]. So you're completely right about there being plenty of spells that don't require a high charisma.
That UA undead warlock would make a pretty good melee alternative for a Hexblade and could make fantastic use of that invocation. You could start at 16 str and 16 charisma, get great weapon master as your variant human feat and focus on your strength. Wont need to touch charisma until level 12 when you can grab lifedrinker.
I'm playing a Pact of the Chain Celestial Warlock with just 14 Charisma, and I'm having a great time. I usually play gishes, and I'm really enjoying this character. Getting to upcast Invisibility and Fly and get the spell slot back on a short rest is fantastic. Whenever we want to do party wide invisibility or party wide flying, the Bard and Wizard can each use a low level spell slot, and I upcast it and get the Fighter and Barbarian covered. With my 21 AC, it's a blast using Shadow of Moil, it makes me a real pain to damage, especially since I can ask the DM "does that spell require you to see your target?" whenever he tries casting a spell on me when I have that up. Shadow Blade was fantastic before I got a Flame Tongue Shortsword. Dimension Door has turned out to be quite useful because the Bard and Wizard both have it, so because I picked it up as well, we can use it party wide as well. And because there's no Cleric in the party, I also picked up Cure Wounds, Lesser Restoration, Revivify, Dispel Magic, Remove Curse, and Greater Restoration. And for my first Mystic Arcanum (which I chose last week), my DM let me take the UA spell [Tooltip Not Found]. So you're completely right about there being plenty of spells that don't require a high charisma.