I doubt there will be Pro-Remain pact in Brecon and Radnorshire ,

Plaid may well have a sincere  in suggesting  a coalition with other parties might be in the offing after Brecon and Radnorshire Conservative MP Chris Davies was ousted by his constituents

Nation Cymru tell us that 

Plaid Cymru leader Adam Price had previously written to the leaders of pro-Remain parties during and after the EU Elections, calling on them to work together to push for a People’s Vote.A Plaid Cymru Spokesperson said:  “The people of Brecon and Radnorshire have spoken and a by-election will now take place. This will likely be a crucial first test for the incoming Conservative Prime Minister.“The two-party status quo in Westminster is breaking down. In last month’s European elections many voters broke the habit of a lifetime by choosing to back pro-People’s Vote parties, rather than the Conservatives or Labour.“Ensuring those of us who want to see a fresh referendum work together, at every possible level, to deliver this aim is crucial. We will explore options for how we can work together, across party lines, to deliver this in the upcoming Brecon and Radnorshire by-election.”

But I doubt the Liberal Democrats , would not be interested  in a coalition and only want Plaid and the Greens to stand down in their favour.

It is of course a Target seat

General election 2017: Brecon and Radnorshire^[10]

Party		Candidate	Votes	%	±
	Conservative	Christopher Davies	20,081	48.6	+7.5
	Liberal Democrat	James Gibson-Watt	12,043	29.1	+0.8
	Labour	Dan Lodge	7,335	17.7	+3.0
	Plaid Cymru	Kate Heneghan	1,299	3.1	-1.3
	UKIP	Peter Gilbert	576	1.4	-6.9
Majority			8,038	19.5	+6.7
Turnout			41,334	76.9	+3.1
Registered electors			56,010
	Conservative hold		Swing	+3.4

Which they have held in the past  

General election 2010: Brecon and Radnorshire^[14][15]

Party		Candidate	Votes	%	±
	Liberal Democrat	Roger Williams	17,929	46.2	+1.3
	Conservative	Suzy Davies	14,182	36.5	+1.9
	Labour	Chris Lloyd	4,096	10.4	−4.5
	Plaid Cymru	Janet Davies	989	2.5	−1.1
	UKIP	Clive Easton	876	2.3	+0.4
	Green	Dorienne Robinson	341	0.9	N/A
	Christian	Jeffery Green	222	0.6	N/A
	Monster Raving Loony	Chris "Lord Offa of the Dyke" Rogers	210	0.5	N/A
Majority			3,747	9.6	-0.6
Turnout			38,845	72.5	+3.0
Registered electors			53,589
	Liberal Democrat hold		Swing	−0.3

Elections in the 2000s

They lost it in the 2015 election after their disastrous gig as David Cameron.s bag carriers

Since then of course they have as of late a revival  and would fancy thier chances in the forthcoming byelection.

However there is a new player in town in Nigel Farage 's Brexit Party Limited Company and Electoral Calculus even predict it being a two horse race between  the Brexit Party Limited Company and the Liberal democrats.

2017 Votes	2017 Share	Predicted Votes
CON	20,081	48.6%	17.0%
LIB	12,043	29.1%	32.1%
LAB	7,335	17.7%	9.0%
Plaid	1,299	3.1%	4.9%
UKIP	576	1.4%	0.4%
Brexit	0	0.0%	30.1%
Green	0	0.0%	5.2%
OTH	0	0.0%	1.3%
CON Majority	8,038	19.4%	Pred Maj 2.0%

The Plaid Green vote may well help the Liberal Democrats to win the seat but they , would clearly like to win on their own back.

With no current MPs and only one Assembly member They are desperate to reestablish themselves in Wales and have eyes on neighbouring  Ceredigion.

It may be in the interest of both Wales and the UK for a coalition defeat of the Brexit Party Limited Company on Brecon and Radnorshire . but it is not in that of the Liberal Democrats and that will prevail.