I'm playing in a campaign where my warlock is permitted to have trained in heavy armor (this is completely separate from the feat). However, when manually adding proficiencies, "heavy armor" as a blanket choice is MIA. I'd just add the individual armor types, but similarly, that doesn't make the HAM feat visible. 

Is it not possible or am I missing something? 

Is your warlock a Hexblade? If it is just give them the Heavily Armored Feat.

They are not, the campaign has a few minor homebrew elements. I'm trying to avoid adding the Heavily Armored Feat as it modifies my STR (which I'd then have to counteract by adjusting my STR modifier to offset the change) if it can be helped

Edit: The more I explore the sheet the more it looks like that's how I'm going to have to handle it :/

Edit Edit: By disabling feat prerequisites I can get around my main issue(s). It still doesn't explain why I can't just add Heavy Armor as a blanket proficiency, but at least I got the results I needed. 

If you want, I have a feat I made in the homebrew section that adds Light, Medium, and Heavy armor proficiencies, its called Fully Armored. 

You can add Heavy Armor as a blanket proficiency either as a Modifier attached to a homebrew, or directly into the Character Sheet itself.

Quote from IamSposta >>

You can add Heavy Armor as a blanket proficiency either as a Modifier attached to a homebrew, or directly into the Character Sheet itself.

 You should outline how you are doing this. I see what the OP is saying, trying to add armor proficiency makes you choose specific armors, not general categories, like medium and heavy.

Quote from Wtfdndad >>

Quote from IamSposta >>

You can add Heavy Armor as a blanket proficiency either as a Modifier attached to a homebrew, or directly into the Character Sheet itself.

 You should outline how you are doing this. I see what the OP is saying, trying to add armor proficiency makes you choose specific armors, not general categories, like medium and heavy.

 Just make a custom feat that grants the proficiency and add that directly to the character sheet under “features & traits.”

Create the feat with the following modifier:

• Modifier—>Proficiency
• Subtype—>Heavy Armor

Save the modifier, and then re-save the whole feat. It might take up to 20 minutes to populate the character sheet so be patient.

Then go to the Character Sheet and navigate to “features & traits” and then click on feats. From there you can click “manage feats” and add your homebrew directly to the character sheet with no need for an ASI.

I hope this was helpful.