Capitalizing a lower case screen name at the beginning of a sentence

When starting a sentence with a lower case pseudonym, such as a screen name of a user account on a website, should it be capitalized? Or are there different cases where it would and would not be appropriate to do so?

My curiosity was sparked by this meta EL&U page where a user comments that he does not mind if his name is lower-cased or not.

Are there any established standards on this practice?

When writing professionally, the first letter in the sentence is capitalized, sole exceptions being when the capitalization could result in a misunderstanding. In such cases, the word is usually typeset differently.

However, when writing in an informal context, you might want to take into consideration the preference of the user. For example, Randall Munroe prefers his username xkcd to remain lowercase; however, as you've linked, some users like nohat do not mind it being capitalized. If you are unsure, I would suggest capitalizing it and adhering to professional writing style.

I would avoid this issue by starting the sentence differently:

The user elufan123 is running for moderator ...

Where possible, I would suggest avoiding putting a username at the start of a sentence, otherwise to pick a style and follow it. Above all, write clearly and unambiguously.

Are there any established standards on this practice?

I asked the editors of the Guardian newspaper's style guide about this:

Q. What's your style on capitalising (sometimes case-sensitive) usernames? And how about at the start of a sentence? Or just avoid it?

A. We spell it however the user does, including at the start of a sentence.