How is 儿化 represented in other romanization systems like Wade-Giles, Zhuyin Fuhao, etc.?
This is not a complete answer, but the Wade-Giles example chart on this Chinese-language Wikipedia entry includes "kêrh" as a transcription of the rhotacized "哥儿." I do not know the general rule.

The GR (Gwoyeu Romatzyh) transcription system, which was used officially in China from 1928-1958, has complex rules for 儿化 spellings. I can't find a complete chart online, but, again, Chinese Wikipedia has some details. GR uses "l" to represent rhotacization, but the precise rules for how the resulting work is spelled differ depending on the syllable final and tone.