GW brought back tournament support and live streaming of games a few years ago, and has openly supported things like the LVO and Adepticon.
I have been playing tournament warhammer since 1999. Around here there would be no issues as long as you cleared it with the TO ahead of time and was clear with your opponents. Any casual who gave you guff would not be worth playing. I feel sad that GW does not support painting and playing with their models in any way their customers want. Thankfully they abandoned tournament support long ago. Pray they keep their greedy mitts off ETC and ITC. RIP Warhammer Fantasy Battle.
GWs only tournament rules regarding painting are the 3 colour minimum/battle ready. Their rules regarding models are WYSIWYG and no non-gw parts, all of which is fair.
You won't have an issue with anyone as long as you are explicit in telling people you use BA rules.
No after market models/parts obliterates 20 years of custom models in my collection. I started as a sculptor/converter. I would never play at a GW tournament. I have paid big money over the years and have Grand Tournament tshirts going back to 2001. Warhammer is not just about paying GW. There are many of us who are alienated by younger players who support the capitalist side if our hobby over the heart of it.
Freedom of modeling in competitive warhammer is important to me. Do not be fooled into selling your independence cheaply for all our sakes.